Guests can answer on the invite page, or by replying to the invite email. We read the reply, and when we can tell what it says, it counts as their answer.
We read the new words above the quoted email and skip signatures. A yes, a no or a maybe in ordinary words counts, and so does a number: 3 of us, party of 4, 2 adults 1 kid. On an event that asks how many kids, the kids are recorded too.
Every reply shows in that guest's messages on your event page, whether or not we could read it. One we couldn't read waits there for you, and the row shows what they said. A reply sent from a different email address than the one on their row is shown but never counted as their answer.
Nothing is emailed back to the guest, and automatic replies such as out-of-office messages are dropped.