Informal invitation to a party/picnic etiquette...

[NMAmy]

If knowing numbers is important, then you should have put an RSVP. So really it is also on the host to be sure that they do the invites the way they want them.

Yep. It's quite common around here for grad parties and open houses to not ask for RSVP's. If I received an invitation without a RSVP, I'd just assume it was an open house and none was required.

If a family member sent me an email asking me to a family party, then yes, I'd reply back.