I don't really get a certain amount of PM's a day.. I get a PM everyday, but some days I could get up to 20. (mainly because I'll be the only mod people see or something)
Once upon a time I used to get ~20 a day, being an Australian Moderator meant that there weren't many other moderators online at the time I was. Also being a market moderator meant I got looooads of requests for changing thread titles