I think the response is typical. I had a friend who threw a b-day party and invited the classmates and none of them RSVP'd nor did they show up which kind of hurt the mom's feelings.
However, he was new to the school and I think kids get so many invites already that parents have to really be choosey as far as what they will attend, since everyone's time is precious, especially on weekends. Sometimes it's possible to get 2 b-day invites for the same week and have parties every single weekend so you have to pick and choose and the parents get a little wiped out. After the new year money can be tight and some parents cannot afford to buy the gifts for various parties either.
Fortunately she invited several old friends and they of course all showed up.
Maybe some of the parents are skittish about the swimming; if their kids don't swim or whatever, you never know.
I agree about trying to get a class list at the beginning of the school year and coordinate with the room mom and that way you can get emails and phone #'s for followup. It's good to get to know the parents in class personally.
In the meantime, have your kids just ask their classmates. My son is the type to just ask other kids if they are attending a party because if he's going he likes to know who all else will be there, at the "social event of the season" type of thing. A lot of the kids will chat at recess about the parties and ask, hey are you going on Saturday?
So your kids can just ask, hey, are you able to make it to my party. Most kids will be blunt and honest and respond yes or no and maybe will say, no, I have a lesson that day or I have another party or we're going to Busch Gardens, etc., then you'll have your answer (or at least a better estimate).
I stopped doing the big b-day parties years ago for my son and we instead choose to take a family trip to Disney to commemorate my son's bday. We just get together with a friend or two for ice cream, cake, mini golf, etc. My son didn't need all those presents and I just felt it was getting too out of hand and overwhelming.
I encountered the RSVP situation that you're describing, but on a smaller scale, where maybe 1 of the 15 parents didn't RSVP and I am kind of type A and extremely organized. And I had invited all friends so not any classmates back then.
I have realized that most people are not as organized and do things last minute. But I like it all planned and done in advance and don't like surprises like what you're encountering.
So his previous parties turned out great, but I did have a parent cancel out and not show and one or two I had to remind personally because they tend to forget even though they gave an RSVP I had emailed them again... you get someone who is sick that day too, etc. You just have to go with the flow. I have heard from a ton of parents at school who invite 40 or more kids to their parties that a lot of them do not RSVP. It just seems to be the norm. Evites are awesome - next time I would get those emails.....!
Oh, someone in a previous post mentioned next time don't give the location of the party, and put on the invite, details will be given upon RSVP. That actually made me smile because you are forcing them to give you an answer because they don't know exactly where the party will be - brilliant...
Good luck - have a fun party!