When you have more than 2500 results in your lead search (or more than 1000 accounts in an account search) you need to split the search into several chunks.
Here is a tutorial on how to do it:
The two splitting filters you can use are:
Function
Seniority Level
By the way is you use these filters to find leads you should stop doing that and only use them to slit your searches.
These filters give irrelevant results and should be used only to split searches.
I explain why in this video: