This answer applies only to Booksy Biz on web/tablet. For help with other Booksy products, contact the Support Team at info.us@booksy.com.

1. Go to Staff Management & Resources
2. Head over to the Resources tab
3. Click on the + icon at the bottom to add a new Resource
4. Set Working Hours for the Resource to match the day(s) of the week you want the assigned service(s) booked
5. Select the service(s) you want to limit and assign the Resource to it

Now your clients will be able to Book only one of the services at the same time.