How would you model this problem?

Suppose I’m trying to predict churn based on previous purchases information. What I do today is come up with features like average spend, count of transactions and so on. I want to instead treat the problem as a sequence one, modeling the sequence of transactions using NN.

The problem is that some users have 5 purchases, while others 15. How to handle this input size change from user to user, and more importantly which architecture to use?

Thanks!!

churn count datascience features information modeling spend transactions