Hi and welcome to the forums!
That’s really up to you to decide. The amount of load you test with is the maximum amount of concurrent connections you’ll know for sure will (or won’t) be handled by your service.
Three things to consider;
during peak - do all users access the same resource? I’ll go out on a limb here and say that likely they are not. If not, what is a reasonable usage pattern for your users? That is likely what you should test.
what is your goal? How many concurrent users do you want to support for? Are you satisfied with your system under test being able to handle 150 concurrent users? If so, then it makes total sense to test that.
If you set it at the monthly total, that would mean that you’re testing whether your system under test can manage the monthly total concurrently, which in your case is more than 17 times your current peak.
I would recommend that you check out the API load testing guide in the docs for a good primer on load testing. And don’t hesitate to get in touch if you have additional questions or thoughts!
Best regards, good luck, and stay safe.