dmah's answer is technically correct - but the point is that you can tell from the per-process figures shown in 'top' how much memory is being shared.
Its not the place to start when working out what MaxClients should be.
There's a complication that the system will use as much free memory as practical for I/O caching - so looking at unused memory does not give a very accurate figure either!
On Linux there's a command 'free' which reports memory usage with and without the I/O buffering/caching, but I don't know if this is available on Mac OS. I used the figures returned from this combined with the number of httpd processes to calculate what MaxClients should be (until it reaches the level where I/O caching is seriously affected, the relationship is fairly linear). But with no procfs I doubt it would be as simple as recompiling it for your machine.
IIRC on BSD in top (or systat -vmstat) it reports the buffer usage along with total memory stats at the top of the page - does MacOS?