The equation that describes the cost depending on the number of jerseys from each company, is:

Evaluate each equation in the given number of jerseys to find which one is cheaper depending on the number of jerseys:
8 jerseys

The cheapest option is Sherman Shirts.
13 jerseys

The cheapest option is Patty's Printing.
3 jerseys

The cheapest option is Sherman Shirts.
10 jerseys

The cheapest option is Sherman Shirts.
12 jerseys

The cheapest options are Patty's Printing and Sherman Shirts.
18 jerseys

The cheapest option is Patty's Printing.