This isn't an exact science, but assuming you're using pre-fork and not MPM inside Apache 2, you should be able to estimate your MaxClients setting using the following strategy.
1) calculate average RSS usage for apache
Do a ps -ylC httpd and average the RSS column using a spreadsheet or command-line trickery. This gives you the average memory size of your Apache children.
2) apply the following formula
(server RAM * MEM percent dedicated) / average RSS usage
Given a server with 10 Gigs of RAM with 80% dedicated to Apache and an average RSS size of 20 megs, and using Google as a calculator:
(10 gigabytes * .80) / (20 megabytes) = 409.6 connections
From there, you could set MaxClients to roughly 400 connections and monitor resource usage. As always, peak load performance must be taken into account as well. Like I said, it's not an exact science; rather, a simple strategy for establishing a baseline value.