If you have an offering with multiple options to select from AND one quota that all options must adhere to, this is easy to set up with sessions!
For example, a course is held on Saturday and Sunday. The course can accommodate a maximum of 20 people, and users can attend on Saturday, Sunday, or both Saturday and Sunday.
1. Create a new session
Go to settings --> products --> create new product --> offering with sessions --> name your session, and set the price. You can add different prices for each specific session option in the next step. 
2. Set the Session Quota
Under the first Settings tab, turn Only one session can be selected "On."
Click "Quota Settings" and enter the quota for ALL sessions in this offering. Our example has a quota of 20. Click "update product"
3. Create your time based sessions
Click Create Session --> name it Saturday (1 day only) --> select the date and leave only Saturday selected --> edit the price, add a time, description, etc. --> update session.
Note that we did NOT include times for each day, but this is an option when setting up your sessions. Click "Add Times" to customize the start and end time for each offering. This is important for sessions that have multiple selections on the same day and you don't want users to select 2 with a time conflict (the system will prevent time conflicts when times and dates are attached to the session). 
On the main products page, click on the arrow by the session folder to see the sessions --> click on the box to the left of the Saturday Only Session --> copy product --> rename it Sunday Only & change the date and day of the week to Sunday.
Copy either of these sessions to make the Saturday & Sunday product, then adjust the dates, days of the week, and price.
4. Test it out on the front end!
Users will only be able to select one, and only 20 users will be able to select any of these options before the offering is no longer available. 
5. Watch the orders come in!
From the products page, you will be able to see the quota, the number of orders, and the number of orders paid for. 
If you click on the session, you can view the orders tab which will show you all users who have ordered the product. From here you can also perform actions on these users, like send them an email, run a report, and more. Here's a great step by step video on creating a session with time slots and quotas: