You need to know what you are reading, and do some junior high school physics, learning about volts, amps, and watts.
ancalotoru:
Nick_Pyner suggested not to use two 9v batteries. But why? I mean, if I need 9v batteries to pulse the solenoid valve, and the Arduino only can get 5v from it's digital output... I need to switch with a relay the 9v power supply.
I said don't use 9v PP3 batteries. You refer to them as "power supply" but this is a contradiction in terms, as they have no power, so put them back where you stole them from - your mother's smoke detectors, the only place they belong.
However, I have seen in the rain bird web page the following information about the valves:
• 24 VAC 50/60 Hz (cycles/sec.) solenoid
• Inrush current: 0,30 A (7,2 VA) at 60 Hz
............
So, what I have try is to regulate the 9V voltage to 7,2V
Bad idea, the result of improper reading. 7.2VA is not 7.2V. The A means something, and you don't need 7.2v anywhere, you already know that you need 24 and AC to boot for the valves, 5v for Arduino, and reducing the number of valves will not save you. Indeed, as I remember it, there is nothing wrong with your wiring and nothing wrong with your code, so, once you understand what watts go where, and where they come from, which isn't from a 9v PP3, everything may be hunkydory.
The 7.2VA is 24x0.3 (VxA)