im not sure about all leagues but i know that china has just 28. ive seen a few asian leagues that have less teams but they may play each other more than twice like in scotland.
 
I have so far found Finland and Iceland play around 22 times
 
If lower divisions count, the Brazilian Serie C has between 18 and 21. Two regional divisions of ten teams, each play twice, then an 8-team knockout to decide promotion (all four semi-finalists go up) and the Serie C title.
 
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