how to calculate least count and why it is multiplied with the readings we get?

Any ammeter will have a range selector.
Suppose the range is selected as 0 to 300 mA.
Count the number of small divisions on the scale.
Suppose there are 30 divisions, then the least count is 300 /30 = 10 m.A
Least count means the smallest division that you can measure using that instrument.
================================
In an instrument a scale reads from zero to 20 A
From zero to 5 A there are 5 divisions .
Hence each division corresponds to 1 A
The least count of this ammeter is 1 A,
================================
Suppose the range is selected as 0 to 300 mA.
Count the number of small divisions on the scale.
Suppose there are 30 divisions, then the least count is 300 /30 = 10 m.A
Least count means the smallest division that you can measure using that instrument.
================================
In an instrument a scale reads from zero to 20 A
From zero to 5 A there are 5 divisions .
Hence each division corresponds to 1 A
The least count of this ammeter is 1 A,
================================

Sorry  I don't understand. You need to explain what 'least count' is and maybe give an example.
Try posting a new question with this information included.
Try posting a new question with this information included.