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I think you need at most 3mm
It's a bit less than that, as the cap itself is 3mm internal depth, and you don't want the steerer to be touching the cap once everything is snugged up. Fork manufacturers vary in their recommendation for steerer projection through the stem, although I think the ones who say 5mm are basing that on the scarcity of spacers smaller than 5mm. Any projection at all solves the 'coning' issue, after that there is a diminishing return as the stress peaks at the top of the stem clamp and fall off rapidly as you go up the free steerer.
@veLLo the caps come with a 30mm long M6 Torx countersunk screw which lies flush with the top of the cap when installed, silver caps get silver screws.
gbj_tester
BTR88
I think you need at most 3mm, but you can adjust with spacers of course. So for example if you have 5mm above the stem then use a 2mm spacer above your stem and put Tester's stem cap on top of that. On the other hand, you could probably get away with only 2mm above your stem although this is not recommended with carbon steerers I think (see Tester's first post).