A May 9 2024 article in PC Mag says no data caps, but that may have changed. I wasn't told about any when I signed up, but I didn't ask as I don't use much anyway:
Comcast Now Offers No-Data-Cap, No-Contract Broadband Nationwide | PCMag
The modem (reconditioned) is given (or sent) to you on sign-up and never needs to be given back.