Can't really put a hard number on it as much depends on the cockbirds' demeanor's.
You might want to have separate coops/runs in case you run into problems.
Single bird integration can be problematic,
unless it's one 'good' and mature cockbird brought into a flock of only pullets/hens.
Bringing another cockbird into a flock with a cockbird already??......
.......whew, could be chaotic and disastrously bloody, even fatal.
I'd stick to the one you have. Start looking at eggs when you break them for breakfast to see how virile your present cockbird is, he may be getting the job done on all of them-if they are all laying.
You could also confine him with fewer hens for a week or two and incubate the eggs produced in that setting.
When do you plan on incubating?