You can't select the number of leads you want to extract from the search within Evaboot.
You must do it through Sales Navigator.
1. Build a list
The first step is to make a lead/account list from the search you have.
Let's say your search has 2500 results, you can build a list of 1000 leads.
For that:
Tick the checkboxes next to the leads/accounts
Click on Save To List
Click on Create a New List
You can only perform this operation page by page, with 25 leads per page. Therefore, you'll need to repeat the process several times to build large lists.
2. Export your list
Once your list is ready, you can export it with Evaboot.
3. Exclude your list from your initial search
Let's say you want to export the rest of the search the next day.
You obviously don't want to export the leads that were in the list again.
Good news, you can exclude them from your search.
For that:
Go to your search
Find the Workflow section
Click on Lead Lists
Find your list
Click on Exclude
After completing that process, all the leads in your lead list will be excluded from the search results.
This way, you can ensure that you won't export the same leads again.