Five is the short answer, but there are a couple of caveats. See the wiki for more info. http://wiki.cohtitan.com/wiki/Set_bonus
Edit: Ah, i need to clarify, you can only receive set bonuses for five, but your question is ambiguous so i feel i should mention that you can still get direct enhancement values from as many as you try to slot.
Damn rule of fives!
It is important to note that LotG is a special case and you can have five LotG 7.5% recharge and up to five of another set that gives you 7.5% recharge since they are considered different set bonuses.
How many of one particular type of IO's (such as Luck of the Gambler) can you have before you receive no more bonuses for slotting that set into other powers? I'm interest in Luck of the Gambler global recharge speed for several defensive powers.