Each class has a specific booking window where you can set how far in advance clients can reserve a spot. This setting is set on an individual class type basis:
Select Classes
Choose the Go button on a plan tile
Select the Booking & Waitlist Settings tab
Choose the edit pencil under the Booking & Waitlist Settings section
Edit the value next to Open Booking Window
Save
The value entered under Open Booking Window will be the number of days ahead from the current day clients can book that specific class.
There is not global setting for this, this setting is controlled on an individual class type basis.
You can also set a Member Book Ahead window.
Select your profile icon from the top right
Choose Business Settings
Select Member Privileges
Set a Member Book Ahead window