Yes, on a competition entry line. The cart draws a quantity stepper on that line and lets you take it up to 5. Each unit you add becomes one more entry in that competition when the payment goes through, so a quantity of three buys three separate places, not one place at triple the price.
The stepper appears on some lines and not others
Your cart shows the plus and minus controls on two kinds of line: a competition entry, and a general product. Every other kind is fixed at one and shows no controls at all.
That includes memberships and subscriptions, and it is deliberate rather than a fault. A membership is one thing attached to your account, so a second copy of it in the same cart would have nothing to do. If you want to cover somebody else, they buy it on their own account.
Five is the ceiling on a single line
A competition entry line stops at 5. The plus control stops responding once you reach it. General products go much higher, which is why you may have seen a larger number on a different line in the same cart.
The floor is 1. To take a line out of the cart, remove it rather than trying to step it down to zero.
What a quantity of three actually buys
Three entries in that competition, all in the registration category shown on that line, all created at the moment the payment succeeds. They arrive on your competitions page as three separate empty entries, each with its own Submit button, and each one can carry a different project, journal or publication.
This is the same outcome as registering three times one after another. It is one payment instead of three, and that is the whole difference.
Each registration category is its own line
A Student place and a Professional place are two different products, so choosing a different category adds a second line to the cart rather than changing the one already there. Each line carries its own quantity and its own price. Read the fee on the line, not the total, when you are checking which category you picked.
If you need more than five places in one competition, check out and register again. There is no limit on how many times you can register, only on how many one cart line can carry.
Before you increase the quantity
Each entry you buy is a separate place you are then responsible for filling. An entry left empty at the submission deadline stays empty, and the fee is not refunded because you did not use the place. Buy the number you are confident you will submit.