There’s no data transfer limit on Cloudflare Pages as long as you abide by Cloudflare’s Terms of Service. Pay particularly close attention to section 2.8 of the Self-Serve Subscription Terms:
Use of the Services for serving video or a disproportionate percentage of pictures, audio files, or other non-HTML content is prohibited, unless purchased separately as part of a Paid Service or expressly allowed under our Supplemental Terms for a specific Service
However, if your site gets a huge amount of traffic (you will know if this is the case), sales might reach out and ask you to enter into an Enterprise agreement.