...but a numeric variable NumberOfToolSetsRented somewhere, that fluctuates between zero and thirty.
Worst case, you have to have a (one-line) table to keep it in. Then you can just update that table (increase or decrease the value by one) every time a toolset gets rented out or returned... And the way to do that is, of course, by using a stored procedure.
Oh, and if you ever find you only have twentynine toolsets on hand, and want to know *which* customer didn't actually return his as he was supposed to, then what do you do?
Then you wish you'd bar-coded your toolchests and used the ToolChestID value of the barcode as a primary key, that's what do you do.