Rsvp

Call or text them. And just let them know if you don’t hear back by such date, you will mark them as declined since you have to have accurate head counts for your vendors.

Call them and say you need to know right now (before you hang up) if they will be attending otherwise they will be considered a decline.

I would text them with a date and tell them that if you don’t hear from them by that date you assume they will not be attending.

I agree with others, I'd text or call them and let them know that if they don't give you an answer by such and such date that you will assume they will not be attending and will notify vendors of number change.

Call once more and (if leaving a voicemail) politely explain that the RSVP deadline has passed and you need to know if they'll be attending by the next day at XX time. If they don't respond, consider them on the not attending aide of things and move on.

If they're extended family, or friends of your parents, perhaps you could enlist their help on calling/texting for RSVP stragglers, as well.