what determines the number of transfers that are displayed there

The account state, which also includes the account history, is retrieved on the page. This history is requested with a limit of 1000 entries. The entries relevant to the wallet are then filtered from these 1000 entries.
The number of transfers therefore depends on how many transfers are contained in these 1000 entries. This is therefore always a different number. If you have more other activities (votes, rewards etc.), there are fewer transfers among these entries.

I hope I have expressed this clearly. If not, I'll be happy to write you an example :-)