First of all, I'd like to affirm--before anyone has even had the chance to post it--that the standpoint that a roleplay isn't really a roleplay if it has mechanics is idiotic. Anyone who has ever said anything to that effect is too stupid a moron to warrant taking anything that they ever said, ever, even half seriously. There is nothing more innately roleplay-ey about freeform roleplays versus structured roleplays. It's as hair-brained an idea as accusing limericks and haikus of not being [i]true[/i] poems.

To answer your question: the ideal amount of mechanisms to add to your roleplay is precisely the amount that improves your roleplay. That quantity is going to be determined by the tolerance of your players to the mechanics, how much of a workload they place on you as GM, etc. There is no perfect anount of structure that always works for every roleplay.