What does your error log (/logs/yourdomain/http/error.log) say about the event?
I just tried to add 'MaxClients 100' to my .htaccess file and sure enough it generated a 500 server error. My error.log showed the following error...
/home/username/mydomain/.htaccess: MaxClients not allowed here
The other directives I've tried work fine in my .htaccess file.
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