The part everybody forgets: counting forty people who cannot be asked
A surprise still needs a caterer's number, a cake size and enough glasses. Guests reply on the page against a name, with plus-ones inside whatever you allow, so the confirmed list fills without anybody being asked directly, which is the only way to count heads for a person who must not know. The names that stay silent appear as a gap, and reminders can go out to them from the list rather than becoming twenty copies of the same message.
Two further notes from people who have done this before. Keep the guest of honour off the list entirely, including any shared household: if a partner lives there, one reply is enough and it should not be the one that reaches the kitchen table. And run the page with someone else where you can, since collaborators can be added with their own access.