Fortunately, in my group, the call is evenly divided from day one. Senior partners take the same amount of call as junior partners and a tab is kept on calls to make sure that everything is being distributed evenly.
For each group, the number of facilities covered, the likelihood of having to cover trauma or OB, etc. factors into the number of calls.
For a 20 man group, you may have 3 or 4 people on call each day. Not necessarily in-house overnight, but some type of late call or home call.
Unfortunately, as the underrated tennis player Johan Kriek stated, there are groups that ravage their juniors with tons of call. Be sure to get a clear answer on how call is distributed before you sign any contract.