lazy-highlight-max-at-a-time

lazy-highlight-max-at-a-time is a variable defined in `isearch.el'.
Its value is

20

Documentation:
Maximum matches to highlight at a time (for `lazy-highlight').
Larger values may reduce Isearch's responsiveness to user input;
smaller values make matches highlight slowly.
A value of nil means highlight all matches.

You can customize this variable.