There is no set number...
Just watch the "primer" on the snap cap. When it starts to get seriously indented -- which will allow overtravel of the firing pin -- replace it.
You'll be surprised how quickly snap caps -- even the good ones -- can wear out. (But its still cheaper than firing live ammo, if all you want to do is speed trigger breakin.)
Note: while most center-fire handguns don't generally require the use of a snap cap, the CZ semi-auto pistols, because of their firing pin retention system, do -- and you're doing the right thing to use snap caps with the RAMI.
I dry-fired my original 75B Military thousands of times without a problem, but busted the roll pin in my functionally similar 40B in a few hundred strokes.
Some of the newest CZs have doubled roll pins, and that might be the case with the RAMI; if so, a busted firing pin retention roll pin is much, much less likely.
