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If you have a 35% equity on the flop; How many people do you need to call your cap/all-in to make it +EV?

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Two.

This is pretty straightforward stuff:
If equity*players (including you) > 1 then raising has +EV.
So so if 1/equity<players then the move has +EV
35%=.35 so
The number of players must be larger than 1/.35 which is just under 3.
With three players (two callers), every dollar you raise should make you five cents.