Hide Unit Quantity removes the option for the customer to select the amount of units they want, when they are at the checkout.
To hide the Unit Quantity selection from the checkout flow, go to Products and select the rental product you would want to hide the unit quantity from.
Select the Options tab and scroll to Hide Unit Selection Field. Tick the box and then click Save.
When the customer goes through the checkout, they will no longer see an option to select how many units they want.