How can we be sure that Open Coin would not create more than 100 billion XRP? ripple server code is still private and beta. 

There is no legality preventing Open Coin to print more XRP now as it is a private company and the network is not at a stage where it is wide scale distributed like bitcoin for consensus process to have any meaning in this aspect.
Doing so would subject OpenCoin to lawsuits due to promissory estoppel. OpenCoin's statement that they will not do so is effectively a warranty. And, of course, it would be a completely self-destructive thing for them to do.

(I am an employee of OpenCoin, speaking only for myself.)