All games i have seen that do this always do in order by number(release date more or less), which seems like the most fair way so no complaints there. So like 4-7 then 8-11 then 12-16 and etc. Naturally they don't merge different regions.

So if you are for example S88 LA you will get merged with either S82 LA or S92 LA, or maybe even more then 2 servers get merged together, i see this on few games.

Might be good time to check out your neighbours if you are on emptier server.