The number of Wi-Fi devices you can have is dependent on two factors: any limit your ISP puts on it, and how many your router can support.
Eero can handle up to 128 Wi-Fi devices.
You’ll have to check with your ISP to see what limits they apply. In my area, with my ISP if you get the unlimited usage, you also get unlimited devices, but that’s not true of every ISP. Of course, that doesn’t mean you can actually have that many all at once, it just means they are not going to restrict you.
The number can add up, particularly if you’re using switches and bulbs. But then, in a small apartment, there’s only so many devices you’re going to have. So it’s up to you.
I agree on the switch protocols: it’s my belief that the reason why historically Z wave was favored for fixed point locations like light switches, and door locks over Zigbee was because of the Wi-Fi interference issues. I think it tends to be especially bad right at the front door where people tend to boost their Wi-Fi so their video doorbells work really well.
But there are some community members like @johnconstantelo who have almost all Zigbee devices and are very happy with their set up, so choice is good. 
Personally, I use Lutron switches. Love their engineering. They work with most home automation platforms. But they are expensive (over $40 per switch, plus you need their smartbridge for the integration) so probably not what you’re looking for.
I do very strongly recommend that anything that will be wired into the mains should have either UL or ETL listing.
FAQ: Does UL Certification matter?
Also, always check the specifications. Quite a few switches are designed initially for the European or Asian markets, and may only support 5 A or 8 A. For a U.S Home, you really want at least 10 A.
(For outlets and smart plugs, look for a minimum of 15 A. There are a bunch that are 10 A and are only intended for table lamps. they can’t handle motorized devices, like a vacuum or a blender, and you won’t always remember which is which.)
As far as other brands, everybody has a favorite, and different preferences, particularly in the form factor. For example, some people want a rocker that you hold down to brighten, others are OK with a separate little button that you slide for that function. Some people care a lot about whether the indicator LED is blue or green, others could care less. So there’s no one best choice, you just have to do some research.
Some of the more popular ones right now:
- Tplink Tapo is the big new brand in the budget space. Matter certified, matter over Wi-Fi for the light switches. Should cost under $20 per switch, maybe down to 15 if you buy a multipack. Inexpensive, and it shows in the engineering and the materials, but they are UL certified, and they’re adequate for the purpose. Read the reviews and you’ll see what the issues are.
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Their sister brand is TP Link Kasa, also kind of a weird form factor, also very popular.
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Meross is another good Wi-Fi brand, but again, kind of a weird form factor.
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In the under $30 category, Zooz has great Z wave engineering, all the latest and greatest, and very helpful technical support. Several different models with different features. Very popular in the forum.
Zooz is the house brand for the retailer the smartest house. You can buy from them or on Amazon.
Zooz also has a really cool scene controller with four small buttons you can use for changing location mode, changing smartthings home monitor mode, scenes, etc. The company provides the custom edge driver you will need.
- Inovelli is another popular Z wave brand in the community, they started out as a budget brand but now have become pretty high end with what I call a gamer aesthetic. Very cool looking, but they do cost more than Zooz now.
Their red series are zwave, their blue series are Zigbee. But they do run about $60 a switch.
- Jasco is a major light switch manufacturer for Z wave. They sell the same model under many different big brand names, including GE, enbrighten, Homeseer, a couple others. These are the ones you can buy at Home Depot.
They are popular, I just don’t personally like the engineering as well as the other brands I mentioned.
https://www.amazon.com/Enbrighten-SimpleWire-Commercial-SmartThings-59337/dp/B0CNDFLTTL/
- Leviton make both WiFi and zwave models. Well engineered, high-quality materials, but on the expensive side. Some of their Wi-Fi devices now offer matter, which is nice. These will cost about $45 each unless you can find them on sale.
https://www.amazon.com/Leviton-D215S-2RW-Anywhere-Companions-Required/dp/B08RSM1KYN/