TOPIC: Which adjectives can we use to mean full booking?

Which adjectives can we use to mean full booking?
1)The booking is full
2)The booking is heavy
3)The booking is ?????
Should the booking(s) be in plural form?
4)The bookings are full/heavy??

Both your first and second examples are fine. Please be aware we also often use"X" is fully /heavily booked" and "X" is booked up" or "X" is booked solid" (= completely sold out you also asked if booking should be in plural form, as in:
4. The bookings are full /heavy?
"Booking " is countable, so you can use it in plural form. Here are two examples from Cambridge Dictionary Online
"Julian was ill so we had to cancel the booking.(singular)
"The show had already taken $ 4 million in advance bookings" (Plural)