invitations

I was wondering how many extra invitations peopel are planning on purchasing? Do you look at list A and list B and do a total like that? or do you just look at list B and add a certain amount extra?
TIA

Add your A List + B List ....

Then add 10 - 15 extra invites because there are bound to be people who have been forgotten from those lists.

Then round up to the nearest 25, because most invites are sold in multiples of 25