Picking that many I would say they all have to be top tier meta picks. Three is what I think on average is a good number of heroes to pick out of all heroes regardless of meta.
Pick only 3 hero pool like me. Ursa, Axe and Slardar. I have never seen those 3 hero ban at the same time. 
looks like 51.43% winrate from here. Which means he is positive 74 games. Which is 2237 mmr. A true grinder.
I'd say it takes about 100 - 150 games to get real comfortable on a hero. Play whatever you want, ideally whatever you think best wins you the game. Theres nothing wrong with playing a bunch of heroes but if you're trying to get really good at specific heroes focus on three at a time until you feel comfortable on them.
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how many heroes should i have in a pool ? and how long it takes probably to learn each hero ?
i want to learn and play on this heroes (midline) , ember storm void spirit dp puck dk ta leshrac kunkka od invoker razzor mk viper , is it waste of time to play so many heroes or i should focus at least 5 or 4 heroes ? need tips guys