Today I was working on the some flow and while testing I have used List Records and intentionally I have not used any filter as I would like to understand the performance. While testing I came to know, the List Records step by default retrieves maximum of 512 records only and the Apply to Each step takes only those 512 records for further processing.
In case if you want retrieve more records, you have to change in the Settings as below.
By default, Pagination would be Off. Turn it on and adjust the limit as per your needs. The maximum you can set is 100000.
Hope this helps.