@WhaleHouseNL I haven't personally set up the Stripe integration before, but it sounds like custom property mapping could help help solve this (it does require Operations Hub Starter).
I'm thinking that you would want to carry something like the Create date from Stripe into the contact record. I'm not seeing anything pertaining to the number of transactions within the dropdown custom property list in the integration page, but you may be able to make a workflow workaround (if you have a Marketing Professional subscription or above) like:
If Stripe Create date is known, increase Number of transactions (a custom property that you would need to create) by 1
Clear the Stripe Create date property value
Enable re-enrollment so that every time a Create date value is set, the contact enters the workflow again
I'm also wondering if you could somehow tie the Stripe transactions to the deals pipeline. If you could create a deal for each Stripe transaction and tie that to a contact record, you'd easily be able to report on the number of deals (transactions) within a given timeframe. It looks like there's a separate Stripe integration with HubSpot quotes that may be worth exploring.