The vendors know what quantities are typically required. So, wheels are sole in twos whereas we often require three. Similarly, servos are solved in quantities of 3, 6 and 12 when sold in bulk whereas we often need 4, 7 etc. That way you definately need to purchase the additional unit.

But go ahead and purchase, it will help you in your next build. For example, I bought 6 servos twice and used 4 each time. Now I have 4 servos spare for my next project and I could purchase only 3 next time

Ameyam