Reduction of a number as a result of entering values elsewhere in a spreadsheet

Hi All! Newbie on this site. I'm an amateur-level Excel guy, so please no code for me!!

I have a spreadsheet that tracks my customers' payments ... a customer typically makes 36-payments in total. Each month, as I enter a payment for that customer (which usually varies a bit), I want the "36" to decrease by "1". When that cell gets down to "1", I will know the customer is making his last payment.

I start with the number "36" in cell-B5.

This customer's first payment goes in cell-J5. When it's input, I want cell-B5 to decrease by "1" to "35".

This customer's second payment goes in cell-R5. When it's input, I want cell-B5 to decrease by "1" to "34".

This customer's third payment goes in cell-Z5. When it's input, I want cell-B5 to decrease by "1" to "33".

..... And so on for "36" payments.

The "payment column" progresses uniformly by 8-columns each time.

Could someone please suggest a formula that I could use in cell-B5 to accomplish this countdown?

Thanks in advance!