Least count of a measuring instrument is the least value of physical quantity that can be measured accurately using the instrument.
Consider a spring balance whose range is 0-25kg.
In its scale, you can see that the 1kg is divided into 10 equal parts. Therefore, the least value it can measure is 100g which can be found out by,
Number of divisions = 10
Least count =1/10 = 0.1kg or 100g
100g is the least count of the spring balance. Similarly we can find the least count of different spring balances of different range.