You have received great responses. I'm just going to add a couple things....
When you send out the email/letter, make sure the parents know that there will be ____ many parties. So, if there are too many volunteers for one, there may be a need for another. There will be certain moms who will want to help with certain parties....
When you ask for helpers, make categories such as....food/drink, cups/plates, games, clean-up/set-up(if needed),etc. This way you'll know who's bringing what and which parents can be there to help and which ones can send in treats with their child
You can also ask if anyone has some ideas they want to share. If you already have ideas, but want more suggestions, tell them that. Say, "I have some ideas I'd like to do with the party games/snacks, but would like any feedback you have" Some of these parents may have been through this before and knows of some great activities that were hits with the kids. There was one game a parent told me about that gained such a reputation as the best game, we did it each and every year until my son headed off to middle school. *You hang donuts from string attached to the ceiling. The kids have to put their hands behind their backs and race to see who can eat the donut off the string first. It's harder than it looks and it geared more for older kids, but it was a whole lot of messy fun.LOL!*
With preschoolers, the simpler the better. Once you get through one party, you'll figure out what works and what doesn't. It can be a lot of fun being class mom!! Good luck and have a blast!!