Mitsubishi Pajero starter

What is the fault with the starters ? pinion wear or what ?

Do what!
7 starters!
That begs belief.
So like the previous respondent asked what killed them and was it all the same death?
Because as far as I can come up with there aren't that many different things that can go wrong with a starter.
It basically comes down to 2,
mechanical or electrical.
So how did they die?

I think it may be electrical cos I go start it and it doesn't click it just smokes a lot and that's that new starter time it will last around 3 months and during that time it starts and runs perfect.

Clean each end of the starter heavy cable,
REALLY clean so bare metal shows through.
And the same with the main engine earth/ground cable,
And where this cable bolts to the engine and chassis.
And I really mean clean,
I would use as a minimum a rotary wire brush.
Reason for all this is you have a high resistance in the circuit somewhere.
It may even be internal to the cable themselves.
But where the cable connect is a good place to start.
Or you could check all the cables/connections for high resistance with a DVM.