There’s no way in the official features to combine alerts from multiple locations. Each smart app only has the access to information from one location.
You would have to do it by using phone calls or texts or emails to IFTTT, and then have IFTTT turn on a virtual switch that represents that specific location and display the virtual switches on something like smarttiles. So there’s only one location ( The community portal) that has all of the virtual switches, The others are just sending in the updates.
So it’s doable, and the end result might actually be OK, but it’s a lot of individual set up work. Also you would only be getting a display result of the alerts, you wouldn’t be able to do anything with them from the portal display.
So as often happens, it comes down to the exact details of what you want. If you just want a display that shows alarm status from 20 different SmartThings accounts you can do that by having each account send a notification to IFT TT, and having that IFT TT account associated with the “community portal” account and turn on a different virtual switch for each of the 20 accounts.