のようなものを変更してみてください...
count(select * from bookings where b.id_customer = c.id)
... to ...
(select count(*) from bookings where b.id_customer = c.id)
のようなものを変更してみてください...
count(select * from bookings where b.id_customer = c.id)
... to ...
(select count(*) from bookings where b.id_customer = c.id)