When I leave apartment and go outside, how do I know which number world I am in, if I don't choose one first?

I'm not sure there is a way to tell without first chosing one, and going to it.

try clicking the world and then maybe it will say what world number you are on

Go to the 20 different worlds. If you remember no one from the first world when you tried the 20 different worlds then I guess you still need to find out by refeshing the app and repeating the process. Do this 3-4 times, if this still doesn't work out for you, try submitting a support ticket to bvg with the title of: Which Server I'm I on?. After so 3-4 business days look back to your ticket. You will read this:

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