You even have 'Offer Manager' option.
This allows you to manually choose which offers will be loaded for every locker of yours.
So you can disable all 'bad' offers and these won't appear when person from that country come to that locker.
Beside that, every locker has number of offers in settings. If you set 5 offers there, then 5 offers will preview, not 10 not all (lol) but if there is no enough of offers from some country, then number of offers might be less, 2 or 0 no offers available from some regions.
If you don't disable any offers in Offer Tools > My Offers or in URL/Content etc lockers > Show Offer Manager, then network will preview top performing offers (number = what is set in settings)
Top performing offers stats are based on the entire network stats. These offers simply perform the best = the most conversions came from them.