A few won't do anything except get pumped back up into the tank. A lot, and I mean a real lot of mb's can cause the pump to cavitate, which can cause even more getting pumped into the tank. It could make the pump louder too.
As far as service on the pump, I'm hoping none...lol. This is the first external pump I used. It's magnetic drive, so there's no seals or anything to change. I guess if anything, just keep the impellor clean. I clean the cup and neck on the skimmer once/week. I change the gfo
in my reactor once/month. I do a 20g pwc
every 2 weeks and the sump is big enough that I can just pull 20g out of and add the new water. I don't have to disturb the main if I don't need to do any "spot cleaning" in it.