Plus one on envelope or RSVP card?

It needs to be on the envelope.

If you only have one envelope, write EXACTLY who is invited on the envelope (Ms. Jane Smith and Guest; Mr. and Mrs. Jane Smith, child 1's name, child 2's name, etc.)

if you have double envelopes, address the outer envelope the primary invitee (Ms. Jane Smith; Mr. and Mrs. Jane Smith) and then on the inner envelope write all the names of exactly who is invited (like above).

If you're inviting kids you would say Mr. And Mrs. John Smith and Family.

For plus ones, If they are in a relationship you need to mention their SO's name, if they are single to your knowledge it would be appropriate to put Mr. John Doe and Guest.

YES put this on the envelope! Name everyone who is invited explicitly. This should help eliminate any confusion about who all is invited. If you only put "ms jane doe" on the envelope she might think the 2 seats reserved is an error.

Thanks!

Following! Thanks I need to start addressing asap

We did the Mr and Mrs John smith and family.

Writing out each child's name is to much on the outer envelope.

Mr John smith and guest for people that are single and both names written out if they're in a relationship.