I am using Firebase Spark free account for my project. The project only require Sign up and Cloud feature. My question is how many users can sign up using this plan with my Firebase project, and what if I delete uploaded files after a regular interval will it still increase storage limit or will go down ones clean data?
For Firebase Authentication there is no documented limit on the number of users that you can register within a project, but there are time based quotas to prevent abuse.
I recommend checking out the Firebase pricing page, and the quota and limits in the documentation for each product.