The maximum number of enemas depends on a lot of things, including how your body works, what the enemas are, and the time period you're going to be doing them. Note: I'll start off with some of the potential problems, but do remember these aren't guaranteed or even common if you're sensible.
One potential issue is the chemicals in the enema solution. Yes, water is still a chemical, so you don't get away from this by saying you just use water 😃. People's colons or other parts of the body may be more or less sensitive to certain chemicals in the solution or absorb more, and you could get chemical colitis, electrolyte imbalance, kidney damage, or a host of other problems. General rule of thumb: the stronger the solution the less often you should take it. Isotonic salt/soda enemas are probably the easiest, Fleet's, heavy soapsuds, and the like are in the stronger class.
Mechanical damage is a commonly reported issue with enemas. Be careful inserting the nozzle.
Another potential issue is the enema retraining your bowels to expect an enema. This is unlikely to be a serious medical issue, but it can be uncomfortable while your bowels adjust to life on their own again.
The best way to handle both of these is to work up and watch your body. Start with a small, simple enema once a week, then move up a bit if you want. As soon as your body starts telling you that it's not happy back off a bit. Things to watch for: All of LisaK's stuff, which indicates electrolyte problems. Any blood in the mucous, which indicates damage to the colonic mucosa (lots of blood or sustained bleeding= go to your doctor), abdominal pain beyond a the normal cramping or abdominal pain that continues. Strange bulges that don't go away. When these happen it generally means you've gone too far, and should back off.
Personal info: I take enemas about every three days because of uncooperative bowels. I usually do 2-3 enemas per session with one soapy, and don't have a problem. I wouldn't recommend starting there, though. Taking an enema a day for a couple days is less likely to be a problem than an enema a day for months, but again listen to your body. Stop for a couple of days and make sure things are still working right, or that you're fine with how it is working. I've heard of people on the boards who take them every day and say that they have no problems and bowel function goes back to normal within 2-3 days, but then again there are people who can't do that. Listen to your body and go slow.