Please forgive me for asking what I'm sure is a common question, but I've checked every faq, site and archive of this list I can find to no avail. Is there practical limit to the number of rules I can add to local.cf?
It's really less limited by the number of rules, and usually more limited by the total complexity of the rules.
Since the rules are in regex syntax, a complicated rule which uses multi-path string matching with lots of backtracing can take as much time to execute as dozens of simple rules, if not a hundred of them. This is going to cause CPU exhaustion.
Of course, if your system is very low on ram, then the total number of bytes of strings to search is going to be a big limiter. However, most heavy-load systems generally have plenty of extra ram (since it's cheap) and generally just hit a "I'm processing mail slower than it's coming in" limit.
Really, since "too much" is a function of amount of incoming mail, CPU and ram resources, as well as rule count and complexity you are probably best off just doing some real world testing and ease rules in while keeping an eye on memory and CPU load.
