Rates differ several times over between countries. That is not generosity to some and meanness to others but a reflection of what the operator earns from a player in each one.
What sits behind a high rate
A solvent audience and heavy competition for it. Traffic in such geos costs more, quality requirements are stricter and checks are closer. The high rate compensates for expensive traffic; it does not hand you the difference.
What sits behind a low one
Cheap traffic and a low average player. Here you work on volume — and here most people start, because mistakes cost less.
How to pick your geo
Not by rate but by whether you can reach the audience. One question: where will I actually find people from this country and speak their language? A rate in an expensive geo is worthless if you cannot reach it.
The easiest route is topical communities — finding chats in your field.
Geo and accounts
Working another country drags accounts and proxies from that region along with it: time zone, language, address. A separate budget line people forget to plan — see registration geo and matching proxy geo.
How many geos at once
One. Two geos together turn analysis into guesswork: you will not know what worked. Add the second once the first is measured.