HAVERHILL — Keni Garcia told police he intended to use the 30,000 bullets they found in his car and home for target practice. That is hard to believe, the prosecutor at Garcia's arraignment said, because if he were to fire a gun for eight hours a day, it would take weeks for him to use all of it. Garcia, who allegedly bought thousands of rounds of ammunition and had 10,000 bullets in his car when he was stopped by police Thursday, was ordered held on $500,000 cash bail yesterday.