I guess you could ball park 5 partners per fund and therefore 4% carry per partner but that's probably not accurate because the carry distribution likely rewards partners in proportion to the success of the companies that they sourced or on whose boards they sit. Also you want to give a little bit to associates and junior partners.
Carry is not guaranteed but if the returns are positive you will get something (and 20% of a middling 5% return is still a lot of money on a billion dollar fund.) In any case carry is valuable in VC because they are like free call options on very volatile assets and option value increases with volatility.
It's difficult to calculate how much VCs are getting from carry because fund returns are closely guarded secrets.
All this said it's pretty tough to break-in to top VC firms.