The earlier the better... You'll realise soon enough that you'll be prompting people time and time again to RSVP (this is based on my personal experience as well as friends' experiences - I've yet to hear of a friend who doesn't have to do this) so the sooner you start (with invites etc), the earlier you'll know the actual numbers and can then confirm your tables and all the seating arrangements (you won't want to stress out over all these at the last minute)... Also, I like using Google Forms to redirect people from FB / WhatsApp to indicate their RSVPs (makes it easy for you to tally all the responses in one place including details like any plus ones or dietary preferences) and this can also cut down on the actual number of invitation cards to print out as you can set your form for people to indicate if they actually want a hard copy or if they're fine with just the soft copy e-invite. Of course, for the less tech savvy people and for really close invitees like family, then I would suggest you still pre-empt them with the details but follow suit with an actual invitation card itself regardless of what their eventual RSVP outcome would be like. Happy inviting and planning!