Many great fighters- Billy Conn, for example- had no amateur experience. The norm seems to have been, at least up until the last 20-30 years, for a guy to have sme amateur experience (Joe Louis had @60 amateur fights) then to turn pro young. Also, it is worth noting that amateur boxing has changed tremendously over the years: in the 1930s, for example, it looked a hell of a lot like thre pro game, and even into the 1970s head gear was optional in some tournaments. Now it is much more common to see guys with well over 100 amateur fights.
As far as a fighter's success as a pro...In today's game who promotes you is more important than a fighter's skill and/or ability; when was the last time an "unknown" came out of no where? While I think amateur boxing is great thing for youngsters to be involved in I alsobelieve that it does as much harm as it does good when it comes to the overall quality of a fighter. Too many bad habits are ingrained essentials of being a good amateur boxer; if you are intending to turn pro it would be wise to find a trainer that will train you in a more pro style from the start. By that I mean teaching you to bob and weave, to use the rolling of the shoulders as a defensive tool, to be economical and effectively accurate with your punches and to punch hard as damage is the game in the pro ranks.
It is a game really because to "make it" as a pro- interms of getting picked up by a manager or promoter that will take care of you, and in terms of getting paid to train and so on- you need to be successful as an amateur, win a few title and what not. At the same time you have to avoid having bad technique become ingrained. And turning pro...Years ago a guy with a great amateur record would turn pro and oveer the years become a much better fighter because he'd be handled by a pro trainer that had the knowledge necessary to fine tune and then to polish a fighter. You rarely if ever see that today- in factit seems that a guy turns pro and that early stage of his career is his peak- he's not only younger but he is fighting regularly and setting down on punches more etc...- and then the deline in skill begins after 5 or 6 years when a guy is still in his mid to late 20s. Two reasons: no trainers out there any more capable of truly polishing a fighter (they don't know enough about the sport) and guys fight twice a yeart, if that, and neglect their skills and conditioning between fights.