what is the least count of the Circular scale or thimble scale

Hi

It depends on how many divisions are there in the circular scale. Normally  there are 100 division on the circular scale. Now one complete rotation (i.e. 100 divisions) of the circular scale makes the cylinder move forward by 1 mm. (see fig.) Thus, each division of the circular scale = 1/100 mm = 0.01mm. That is the Least count of the circular scale.