Hi,
I had same question - I do not want them to purchase more than 1 service.
I do not know what version I have, but the instructions someone wrote for all those 'brino, etc. versions', did not work for my version but - Thank God - these steps listed below did fix it.
Home page
go to 'Commerce'
go down to 'Checkout'
go to 'Service Product Limits' and 'uncheck' this box. yeah! This worked for me, hope it works for you.
SERVICE PRODUCT LIMITS
Allow customers to buy more than one service product at at time.
Let Customers Buy Multiple Service Products
J.Stone from StonepointDS 4/7/22