history-length

history-length is a variable defined in `C source code'.
Its value is

100

Documentation:
Maximum length of history lists before truncation takes place.
A number means truncate to that length; truncation deletes old
elements, and is done just after inserting a new element.
A value of t means no truncation.

This variable only affects history lists that don't specify their own
maximum lengths. Setting the `history-length' property of a history
variable overrides this default.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.5 of Emacs.