Generally for a world game, I'd say you'd want at least 6, and not more than 20... which is the limit. Honestly, put as many sides as you darn well want, you'll get enough players if you get on at that time when the player count in the lobby is over 40. 12 is perfectly fine for a world scenario.
A scenario with fewer players is actually screwed over more when people leave; if you have a scenario with 5 guys, and one leaves, you've lost 1/5 and are forced to restart. Every scenario has the issue of leavers, not just ones with many people; if you want to prevent that, kick people who you've tagged as leavers, or just set rank limits. I usually set rank limits to minimum 4, just to be safe.
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"If in other sciences we are to arrive at certainty without doubt and truth without error, it behooves us to place the foundations of knowledge in mathematics."
-The Opus Major of Roger Bacon