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What to ask
Keep it short: name, attending or not, and number of guests. Add a menu choice only if your venue needs it.
When to set the deadline
Check when your venue and caterer need final numbers, then set your deadline before that date so you have time to follow up.
What to do about guests who do not reply
Send one friendly reminder near the deadline, then call the rest. Guests who phone you can be added to the same list by hand.