A regulator is kind of like an administration program for the actual server.
There are many out there, unfortunately I have only used one of them called brenbot.
Brenbot utilizes an IRC channel and ingame to accept commands to control/modify server.
Within brenbot, there is a file called mapsettings.ini
It is layed out in an xml type manner
I.E.
<map name="C&C_Arctic.mix">
<time>45</time>
<mines>10</mines>
<vehicles>8</vehicles>
<donatelimit>1</donatelimit>
<rules>Do not base camp in back of enemy base. Do not attack Turrets.</rules>
As you can see, edit that file and you have different limits.
There is also a command that allows you to change vehicle limit within irc or ingame if you have administrator status.