If your event is providing a T-Shirt option through the Standard T-Shirt question, there are a couple ways that you can access numbers.
1. Under the Registration > Reports section, access the Standard/ShirtSizes report to find a count report giving the number selected for each size and Gender -
*If this report does not already exist under the Reports tab, you can add it by clicking on +Add Report > Standard/ShirtSize.
2. If you wanted to see the shirt sizes in more detail (including athlete's name, shirt sizes within a date range, other information related to the athlete) you can use the Standard/ConfirmedEntries report.
The T-Shirt option will be found under the Registration section of the Confirmed Entries report -