How many arguments are allowed in a sum formula?

A minimum of one argument is needed and you can have up to thirty. Though the answer would be obvious, you can use a single value in a SUM function like this:

=SUM(5)

You can also use a single cell:

=SUM(A23)

It can also be another calculation or function, though these can be done without using the SUM function:

=SUM(A5*10)

=SUM(AVERAGE(A24:B30))