SQL STATEMENT TO INCREMENT COUNTER BASED ON CRITERIA

I am buiding a view in SQL with information coming from two files.
File A has
Invoice Number
Flag if it has a Summary Invoice Number
File B has
Invoice Number
Summary Invoice Number

I want my view to have
Invoice Number, Summary Number, Counter

I want the counter to increase each time the same summary number is used.  If there is no summary invoice number the counter would be 1.

Example
Row 1
Invoice Number = 1
Summary Number = 100
Counter = 1
Row 2
Invoice Number = 2
Summary Number = 100
Counter = 2
Row 3
Invoice Number = 3
Summary Number = 200
Counter = 1
Row 4
Invoice Number = 4
Summary Number = NULL
Counter = 1


Can anyone help with the code for the counter