Question

if most of curretnt passes through shunt how ammeter measure current

Answer 1

Total current through ammeter will be the sum of 2 currents, 1 going though the shunt and the other going through the galvanometer (lets call it that, so we don't confuse it with the ammeter as the whole circuit).

So you can say that: \[\ (I-I_v)R_s =R_vI_v\]
From this you can calculate the total current passing through the ammeter if you know all the other variables.
Your galvanometer measures the current I_v, R_v is it's internal resistance and R_s is a known shunt resistance.
So now you can calculate the total current passing through the ammeter.
That's a lot of work to do, so instead of calculating I (as in current) every time from this expression, you can make a scale on the galvanometer that will do this "calculation" for you.
In other words, the galvanometer measures the current I_v but it's scale is made in such a way to show the total current I.