you're only alowed to carry 8 pasangers..... if you're doing it for money or as part of your job
you're alowed more (16iirc?) if they're your family... you're doing it free for a charity / shcool trip etc...
It's the other way round. You can carry up to 8 passengers on your driving license (in general) - that means 9 people including the driver.
BUT - and this is valid in the UK ONLY
If you had entitlement to drive cars prior to 1 January 1997 you can drive a minibus provided you are 21 or over and the minibus has a maximum of 17 seats including the driver's seat and is not being used for hire or reward.
Having said that - to drive a minibus which has nine or more passenger seats for hire or reward
you will normally need passenger carrying vehicle entitlement (PCV). To obtain this you must meet higher medical standards and take a further driving test. However, if you drive a minibus for an organisation under the minibus or community bus permit scheme you will not need to have the higher PCV entitlements.. You can NOT carry more than 8 passengers OUTSIDE of the UK AT ALL
All the details are available here
I was looking into it, as my bus is registered as a 12 seater (and I thought I can only drive 9 seaters on my license...).
The insurance is different issue. If your van is registered as a 3 seater and you add seats to the back (now forgetting all the "stay alive" risks related to it) you instantly become illegal. Because you just "modified" your vehicle and your insurance is invalid as someone said before me.
My friend builder did solved the situation with fixing and plating rails for triple bus seat in the rear and registered the vehicle as 6 seater. Insurance is fine with it as long as the seats (in this case rails) were bolted in and plated properly (certificate needed). Now he has the option of putting the bus triple seat in when needed (it has got the seatbelts on it). Not sure what else was needed though...