<IfModule mpm_worker_module>
StartServers 1
Serverlimit 1
MaxClients 2048
MinSpareThreads 2048
MaxSpareThreads 2048
ThreadLimit 2048
ThreadsPerChild 2048
MaxRequestsPerChild 32000000
MaxMemFree 64000
</IfModule>

にしたら、configtest すると、

Performing sanity check on apache22 configuration:
WARNING: ThreadsPerChild of 2048 exceeds ThreadLimit value of 64
threads, lowering ThreadsPerChild to 64. To increase, please see the
ThreadLimit directive.
WARNING: MaxClients of 2048 would require 32 servers,
and would exceed the ServerLimit value of 1.
Automatically lowering MaxClients to 64. To increase,
please see the ServerLimit directive.
Syntax OK

となるようです。