Most Commercial vehicles have between 2/3 seats as of standard.
On occasions you will come across, crew cab vans and double cabs. These vehicles usually have a total of 5/6 seats and come out of the factory built this way.
Insurers usually charge more or decline insurance for vehicles with more than the traditional 2/3 seats. The reason being, is more people in the vehicle leads to more driver distractions. There is also much greater increase in personal injury claims.
Additional seats/removal of seats in the vehicle usually need referring or checking against the referral guide. The customer usually needs to update the DVLA with the change so they can correct the log book.
Sabre will quote up to 9 seats as of standard on CDL. 9 or more seats get classed as a minibus and need to get referred to the Technical team.