Each time you add a new repeater, keep track of how many Sengled bulbs you can add before you hit the limit again. Then note that that model of repeater can support that many “children.”
If you come back and share that information with the community in this thread, that would be great, but in any case you’ll have a good idea about which repeaters will be most cost-effective for you. 
I just wanted to thank everyone for the helpful information in this thread.
For future Googlers of this problem, there is a similar problem with Amazon’s EchoPlus. The EchoPlus can only handle 6 Sengled Bulbs. Such a problem can be found in this Amazon review, and Sengled confirmed this to me by phone.
After discovering this, I bought a SmartThings Hub. Which I discovered has a 32 bulb limit. It is very useful to know that a repeater will let me add 6 more bulbs. Any recommendations of brands would be appreciated. I only need 10 more bulbs more to complete the house.
Are you in the US or the UK?
If you are in the US, The SmartThings branded zigbee pocket socket is currently on sale at almost 50 percent off, and it’s definitely worth considering:
https://www.amazon.com/Samsung-SmartThings-Outlet-F-OUT-US-2-Amazon/dp/B073GV2PQY/
Over the last year, the most popular has probably been the Lowe’s Iris pocket socket, which functions as a zigbee on/off Device and a zigbee repeater, but also has a Z wave radio inside it which works as zwave repeater. So you are getting two repeaters in one device, which is very unusual, but also very helpful.
It’s often on sale for around $25, but unfortunately is not on sale right now.
https://www.lowes.com/pd/Iris-120-Volt-White-Smart-Plug/999925330
So for the US, both of those are good. But I don’t know what the child maximum is for either – – if you get one and find out, come back and let us know.
If you’re in the UK, right now it’s much trickier.
I have smartthings outlet and a cree connected led bulb. They work fine as repeaters. I would recommend smartthings outlet as they work fine and were recently on sale as well for $20. I just got the delivery. Also picked up a lowes smart outlet. Will be doing some testing tomorrow to check the total devices each repeater would let me add and update here.
Thanks for both of these replies. I will look into each suggestion!
One question. Can a second Smart Things hub be paired or connected to a first? My thought here is that if a repeater only gets me 6 more connections (for $22), it may be more cost effective to get a second hub and 32 connections for $50.
No.
Smartthings only allows for one hub per “location”. Two hubs will not repeat for each other or be able to see the devices that belong to the other one.
https://support.smartthings.com/hc/en-us/articles/115003226203-SmartThings-Hub-Miscellaneous-FAQ#two
Hello Tom @tpmanley,
Thanks much for the information. And the hope that someday we’ll get a bump to 64. (I’m already at limit because of some Xiaomi stuff that won’t work through repeaters - I know they’re not supported but I love the form factor and price!).
Anyhow - is it 32 for Zigbee plus x for Z-wave or 32 all in? I’m guessing the first but just wanted to be sure.
Thanks!
Alec
It is still the goal to increase the limit to 64 as long as we can prevent that unrelated regression I mentioned before. Unfortunately we don’t know yet when that will happen.
You guessed correctly, it’s just for Zigbee. If you can add routers then you can add capacity for more end devices.
The maximum for Z-Wave is 232.
For those unfamiliar with zigbee terminology, “routers” in this context just means zigbee repeaters. That includes most of the mains powered devices that can work with smartthings, such as zigbee light switches, pocket sockets, relays, and plug in sensors. ( zigbee repeats only for zigbee and zwave repeats only for Zwave.)
Hiya Tom, @tpmanley
Thanks - nice to know it’s still on roadmap for whenever. Everything else I have works fine w/ routers nicely (even my 20 old Quirky Trippers!) .
Best,
Alec
Good I found this topic. I have a ticket open for following issue
For a couple weeks I’ve had trouble reconnecting Xiaomi devices that suddenly dropped off. I guess this kinda explains why.
Anyone has a reference for a zigbee repeater supporting Xiaomi in Europe?
I guess for now I will try to reduce the amount of sensors.
Have 42 Xiaomi zigbee devices paired with 6 that dropped off. This doesn’t include the smartsense multi and socket.
+1 on the 64 limit !
Thanks
Jean.
The math works… at least now we can stop trying over and over… 
Interesting feedback from support, somewhat contradictory to another post mentioning to turn off all other zigbee routers before connecting Xiaomi devices.
Ahh, we have noticed that other users have had similar scenarios with the Xiaomi devices where it’s much easier to connect when a Power Outlet is already connected. It seems like the Xiaomi devices don’t follow the standard ZigBee protocol from our experience.
Yep, you are correct that there is a limit of 32 directly connected devices. There are plans to increase this to 64 devices but we don’t have an ETA at this very moment. The best thing is to stay tuned to our Blog for all the latest news and updates. As you mentioned, adding extra Outlets as repeaters can allow you to add more devices so you may want to take this into consideration.
Not entirely contradictory - if you have slots left on your hub, you do want to turn off all the other routers/repeaters so that you can confirm pairing to the hub. If you have no slots on the hub, you can connect to a repeater - but at least for me it just doesn’t work after the initial hour or two (never switches into the reporting every 55 minutes mode).
I’m playing around carefully with disconnecting a few of my devices which I know work well with repeaters (like my Quirky Trippers) to clear out a couple of ‘direct’ slots and see if I can pair my remaining 2-3 sensors. Idea being that I pair the Xiaomis to the direct slots, then repair the Quirkys to a repeater. I’ll see how it goes.
For SmartThings approved zigbee devices that would likely be fine but for Xiaomi devices I had at least 8 open slots and they still would always try to pair through a relay device and then eventually fall off.
Only way was to clean out remove all Xiaomis devices, kill power to all zigbee relay devices then pair the Xiaomi devices and then re-power on all the zigbee relay devices.
Hmmm. I have one device that can’t be seen at the moment.
It’s in a room with 2 repeaters though… probably just a battery… will swap batteries with another device to validate. If not, I’ll have to start looking into the zigbee limits.
Yeah - of course will turn off the zigbee repeaters - just have 4. Fortunately most of my zigbee bulbs are off on the Hue hub - I think I can get by without disconnecting those since slightly different protocol.
So the question is: will I be able to pair more than 32 Xiaomi devices (to stay connected I mean)?
I have started removing sensors now (still had some spares connected - I was pairing them in batches so I always had some spares that were stable). I hope I can get below 32 without impacting the automation level.
It’s not looking like it