what is the minimum number of hands to look at when using this stat and what is the ideal number of bb/100 that a average player gets?

I see the 10,000 number thrown around a lot. Your bb/100 will vary based on what limit you play. Again, a general number I've heard people throw around for 2nl is minimum 10bb/100. That will get smaller as you go up in stakes

It'll vary a lot based on different factors from player to player which includes aggressiveness of the game, style, how big of a winner/loser you are, game selection, and of course, variance.

200k hands make quite accurate estimate

The greater the sample size, the more accurate the winrate obviously. 200k is a good sample size, but 10k is enough to get an idea. Even 0 is good number, it will make you a breakeven player. Everything above 0, makes you a winning player which should be your only goal. As a winning player you belong in the 5% of online players. It's more important to improve your game than looking constantly on your winrate. Sure it's something you want to keep track of, but take this as a advice an stick to a good solid game style, an the results will come. I'm currently winning in NL5 after over 50k hands. I will consider moving up after I can say I beat the limit, and that will be when I have grinded 20 buy-ins, hopefully next month

I am moving up when I won 40BI.. it happens to me in past, that I won 20 BI during 8k hands on heater.. so I became more conservative about moving up. What do you think about it?