Handling software Licenses

Hello All,

We have the below scenario:

We need to handle the purchase of softwares.

Ex. First we will receive a consolidated agreement price from vendor for the cloud storage. Depending on the usage the price may increase or decrease.

Do we need to create Po's every month or one PO with 12 lines for the year? If we create PO how do we adjust the price on monthly basis?

Or do we need to maintain an agreement and release it every month?

What is the standard practice to capture this scenario?

Thanks in advance,