It is a mistake to hire huge numbers of people to get a complicated job done. Numbers will never compensate for talent in getting the right answer (two people who don't know something are no better than one), will tend to slow down progress, and will make the task incredibly expensive.
My biggest mistake is probably weighing too much on someone's talent and not someone's personality. I think it matters whether someone has a good heart.
Talent is extremely important. It's like a sports team, the team that has the best individual player will often win, but then there's a multiplier from how those players work together and the strategy they employ.