Hi Sam. Currently there is no way to limit how many future appointments an Existing Patient can make online. It is possible for them to login to your Patient Booking Gateway and book multiple appointments within the boundaries of your availability and your Maximum Search Range e.g. 2 months into the future. They will also be limited by your Lock Out Defaults, though once the lockout period has elapsed, they will be able to book future appointments once again.
You can however limit a New Patient to only book an initial consult. Until they've attended their initial consult they will not be able to book a subsequent appointment. To check that this is setup correctly, head to File > Patient Booking Gateway Configuration > Guest Bookings and ensure that the follow option is unchecked: