The new Apache versions have renamed MaxClients to MaxRequestWorkers, but they
support both names, they modify the same limit.

The error message when the limit is reached is this:
AH00161: server reached MaxRequestWorkers setting, consider raising the
MaxRequestWorkers setting

I suggest the default configuration looks something like this:

# number of simultaneous requests that will be served, correlates with
ServerLimit
# was called MaxClients in older Apache versions
# http://httpd.apache.org/docs/2.4/mod/mpm_common.html#maxrequestworkers
MaxRequestWorkers 150

The problem that I faced is that I was setting MaxRequestWorkers to a higher
value, as the error message was suggesting, but MaxClients was set to a low
value a few lines down. This was overriding my value.