Sorry, but your question is confusing and incomplete.
Perhaps you wish to know how to determine how many days there are in a
month. If so what month? How does Access know which month? Which year?
Let's assume the current month.
You can use the DateSerial function. Look it up in VBA help files.
The day 0 of the following month will tell you how many days there are
in the current month.
=DateSerial(Year(Date()),Month(Date()) + 1,0)
Change Date() to a field name [DateFieldName] if you want the number
of days in the month of a date stored in a date field.