As T-Shirts were an optional extra purchase, rather than include the t-shirt option for each participant we included this in a separate section for merchandise.

To do this we first set, under Common in Group Reg. all of the corresponding fields to Yes, but save only for first registration + at the end of the entry form". The data is therefore only stored for the first registration which ensures later statistics on number of T-Shirts are not multiplied by the number of participants per group. 

The option to purchase a T-Shirt was setup in 2 separate fields, a CheckBox for Single Registration and a DropDown for Group Registration with options 0;1;2;3;4. The GroupTShirts DropDown was included twice in the form fields setup, each with different rules.

We only wanted to allow families to purchase a maximum of one t-shirt per registration, this would be either 3 or 4 t-shirts maximum. We showed the dropdown according to how many registrations were in a group, which are defined before the T-Shirt section. This is achieved using Show only if set to use the Number of Lines, which relates to how many registrations in the group. 

For the field showing where Number of Lines = 3 we also applied a validation rule that this can only be less than or equal to 3.

val([GroupTShirts])<=3

Since Additional Fields are stored as text we had to use val() which converts the text to a number.

We finally capture the 4 sizes, each setup as a unique Drop Down additional field. Each set to show only if the value of the Number of T-Shirts is larger than 1 less than that T-Shirt number. 

e.g. T-Shirt size 3 needs to show if they are order 3 OR 4 T-Shirts so it shows if the number of T-Shirts is greater than 2.