How to create conditional logic in forms based on number of respondents?

long time lurker/first time poster, here with a potentially basic question:

I'm setting up an online questionnaire and I need to automate responses to the participants based on number of participants.

To clarify: the first 10/15/X number of participants should receive one type of confirmation message, the following participants (X+n...) should receive another confirmation message. I only need to segment the respondents into 2 batches / 2 different confirmation messages, but I'll be working with hundreds of respondents so I'd rather automate the process.

Basically, I've been looking into gravity forms, google forms, etc, and conditional logic for confirmations, but all of these are based on the participants' responses, not on the volume of participants.

I'm sure this is really basic, but my belt in Google-fu hasn't helped me so far.