[Exim] Force maximum recipients number in ACL

Hi,

because I do not know where to start my search and which terms I've
got to use I will simply put my question here - being sure this has
been discussed here already.

I want to regulate the number of maximum recipients in the h_to
field let's say to three whithin the smtp-session (via acl's).

Where can I start my search and for what terms do I have to search??

Thanks,

Michael