The Leads page shows your leads in pages. The Items per page selector at the bottom of the list sets how many leads each page shows.
Why the number changed
Your choice is saved in your web browser, not in your LetHub account. Clearing your browser's cache and cookies removes it, so the list goes back to 20 leads per page. The same happens on a new computer or a different browser.
Set it again
Click Leads in the left menu.
Scroll to the bottom of the list.
Open the Items per page selector.
Choose the number of leads you want per page.
Your browser remembers this choice until its cache is cleared again.