Party Invite minefield- what's the etiquette?

[threelittlebabies]

Ok, I am a "new girl", that is, my ds started nursery a month ago. He has already had one party invite for next weekend to which his entire nursery class (20 children) has been invited. The children all came out clutching their invites at home time one day.
Ds' party is the weekend after next. The venue we are having it at has a maximum of 20, and as he already has friends to ask, we cannot invite the entire class. I don't know whether to just invite 2 or 3 of his new nursery class, or not ask any. I can't bear the thought of the other children not getting an invite but seeing that others have, and also I don't want ds to be upset in the future if other don't invite him. I have tried explaining to him that we can't go to everyone's parties, but he doesn't really get it (he is nearly 4)
I know I am fussing over nothing, but help! Someone tell me what to do. I know I'm a big wimp but I just want ds to make friends and be happy at nursery. Can't believe I am having a dilemma over this!
Oh I asked his teacher, but she was quite vague

When DD was at day nursery we used to ask the nursery staff who DD played with most, and invite a set number of her most played with friends. Can you hand them out discreetly? Have the teachers put them in bags or something?

Dd has been invited to many class parties but also more selective ones. We have a limit on numbers due to the activity (swimming) and after her brother and cousins she was able to choose 7 children out of a class of 20 (we figured on a couple of refusals)

The teacherfs do tend to put the invites inside the bookbags though so you only find them when you open them. Some mums open them at the gate but I always wait til we get in the car.

Oh hula, they aren't allowed bags, they "don't have room for them", apparently. Don't get me started! And his teacher was infuriatingly vague/unsure about who he is friends with, which made me worry he doesn't have any friends! [mad paranoid bint emoticon] He tells me absolutely nothing about school.

Considered handing them to mums in a covert manner, but I know the others would find out anyway. Either that or make dh do it on one of the days he takes them!

Thanks for the replies you two, am going to sleep on it