Need help with team composition!!

i'm a little confused with the team composition

1. I've seen guides that say team must have only 1 carry,but i see alot of games with 2-3 carry

like in this video
navy vs quantic (navy use riki+broodmother)

http://www.youtube.com/watch?v=t8z6AC1MZ6M&feature=related

Someone can tell me why they use 2 carry?

They don't use one of them as a carry. They used the Riki as a carry, while they played Brood as a pusher not a carry. In many games there are 2-3 or even 4 carries, but not all of them are played as carry's. For example, there is a lineup of the following:

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Lone Druid
Broodmother
Invoker
Leshrac
Jakiro
Shadow Shaman

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In this certain lineup, if it were a competitive match, Lone Druid would be played as the extreme hard carry, whereas Brood, with the carry role, would be played as the annoying pusher type. Invoker is played as a carry a lot in competitive games, yes but in this case, they would either not pick him or pick him and play him as a support/ganker. The rest are as normal.

In pubs, however, a lot of people seem to love carry's and in a lot of them there are many carry's going for the same role:carry.

That help you?

thx alot!!

Besides what wild said, 2 carries can also be useful, let's say you have 2 carries and the enemy team has one, their team is based on 1 carry to 'win' the game, while your team has 2 carries who can shut him down and win easier.