i have seen some links in comments in blogs. all i want to know is does these links from comments act as back links.

Does google consider these links for page rank?

Please help me in this regard.

Advanced thanks

See this wiki, It might gives you a clue with the small diagram

http://en.wikipedia.org/wiki/PageRank

the number of IBLs is very important to a site!

but more important is that ... does the spider bots really discover all the IBLs from a specific site ?

well... that's the blogger's or forum's webmaster job. This is usually set-up from the administration panel.

All in all here is a nice helpful table on how many links a site needs to get a desired Page Rank?