diary-number-of-entries is a variable defined in `diary-lib.el'.
Its value is

Specifies how many days of diary entries are to be displayed initially.
This variable affects the diary display when the command M-x diary is
used, or if the value of the variable `calendar-view-diary-initially-flag'
is non-nil. For example, if the default value 1 is used, then only the
current day's diary entries will be displayed. If the value 2 is used,
then both the current day's and the next day's entries will be displayed.

The value can also be a vector such as [0 2 2 2 2 4 1]; this value says
to display no diary entries on Sunday, the entries for the current date
and the day after on Monday through Thursday, Friday through Monday's
entries on Friday, and only Saturday's entries on Saturday.

This variable does not affect the diary display with the `d' command
from the calendar; in that case, the prefix argument controls the number
of days of diary entries displayed.

You can customize this variable.