It is given on your website in JEE Syllabus that in Physics, we have least count. What is this? I never saw it in HCV or Brilliant Material. Can you suggest any Book or website where Least Count chapter is there?

Least count of any instrument is the minimum quantity you can measure with it.

For example, with the commom ruler(scale) you get in market, you can measure a minimum length of 1 mm. So its least count is 1 mm. There are good conceptual questions on least count in NCERT books of Class 11 (they had in 2004, I dont know if the new editions also have it)

Apart from that, least count is a vey easy topic. Even if you just practice the questions which appeared in last 10 yrs JEE exam, that is more than enough.