|It sounds like your second question calls for VLOOKUP.|
e.g. with your Client Numbers in TabName!A1:A5 and your Client Names in TabName!B1:B5, you would use:
Read up on VLOOKUP in the Excel Help files to see how it works.
As far as your first question, I'm not sure what you are looking for.
In your first post you said:
i need to see, how many payments were made
"How many" - to me at least - sounds like you want to count how many payments were made.
However, in your second post, you said you tried:
=IF((WeekNum) ...??? SUM(Payments))
The use of SUM tells me you want to add something.
What is you are looking for - a count or a sum?