How many prims are we allowed to use for building vehicles in SL?

[Damien Avedon 2]

That depends.  For Physical movement vehicles, work at a maximum of 30 total prims.  Sometimes, you can go as high as 32 prims, but that isn't a given.  30 prims is safe and they'll stay together when you cross a sim border.

Getting into Non-Physical movement, you can go as high as you like as long as you remember that A: Crossing sim boundaries can make your vehicle fall apart, and B: non-physical movement scripts can get you banned on some regions because of the lag they cause.

Some vehicles are banned simply because of their high prim count and script load.  That means your vehicle and you may be kicked off a sim without warning because it is using too much sim memory and causing lag.  Some vehicle/residences are allowed if the movement scripts are deleted, but don't count on it.  Some sim admins won't bother to count scripts, they'll just return and ban and let you cry to the sim owner later.

Bottom line:  30 (Thirty) prims is the practical limit for vehicles... and remember to count the base prim with all the scripts in it too!

[Damien Avedon 2]

That depends.  For Physical movement vehicles, work at a maximum of 30 total prims.  Sometimes, you can go as high as 32 prims, but that isn't a given.  30 prims is safe and they'll stay together when you cross a sim border.

Getting into Non-Physical movement, you can go as high as you like as long as you remember that A: Crossing sim boundaries can make your vehicle fall apart, and B: non-physical movement scripts can get you banned on some regions because of the lag they cause.

Some vehicles are banned simply because of their high prim count and script load.  That means your vehicle and you may be kicked off a sim without warning because it is using too much sim memory and causing lag.  Some vehicle/residences are allowed if the movement scripts are deleted, but don't count on it.  Some sim admins won't bother to count scripts, they'll just return and ban and let you cry to the sim owner later.

Bottom line:  30 (Thirty) prims is the practical limit for vehicles... and remember to count the base prim with all the scripts in it too!

[Lindal Kidd 17,923]

Lots of people get around the 32 prim limit by making vehicles in two parts.  1) a very simple physical vehicle...just a transparent outline and the sit poseballs...and 2) a more complex object that the driver wears as an attachment.  As long as the driver doesn't stand up, the two parts appear to be one whole vehicle.

[Innula Zenovka 3,504]

If you use the Land Impact system of accounting, the important figure becomes not the number of prims but the physics weight of the vehicle.   So, so long as you keep the physics weight down to 32 or lower, you can use lots more prims by setting them to physics shape none.