people can vote sending clam, but can someone send many times the same coins?
No.
You can't support a petition by 'sending' CLAM.
Supporting a petition requires staking a block.
But need 50% of votes for change? So if jd have 750k clam votes change will happend?
There is no specific threshold of what votes are "needed". Nothing automatic happens as a result of the petitions (unlike the case where a fork is deployed in the code and waits for a certain number of block votes to activate). It is a poll. After that, developers, users, stakers, service operators, etc. will all decide what to do having been informed by the opinions of the current (at least during some past time window) stakers.