how much money do you need to become a stock broker?

Hmmm... interesting question, but perhaps you are a little confused about what a stock broker typically does.  A stock broker is generally the person who executes a trade on behalf of a client while working for a brokerage firm.  So you don't really need money of your own, per se - you don't even need much experience to get started, as many firms will let you work just for commission to start.  Once you build up a client base and get some experience under your belt, then you can start making the kinds of big broker salaries many of us hear about - and potentially quite a bit more, if you have a natural talent for it.

Now it occurs to me with your question you may be thinking of a "stock trader" - where you have someone else broker the deal but you are the one investing the money.  In that case, really any amount can work but obviously the more you have, the more you stand to make.