1) How many simultaneous solenoids can I activate?
As long as your input power supply has sufficient power rating, you can activate as many as you want. Each zone has a 800mA max current limit. There is, however, an on-board 2A fuse. In case you activated too many valves that triggered the fuse, you can always solder a wire to bypass the fuse.
2) The images online show a set of blue screw terminals for the mini relay contacts.
I don’t have the part number. These pins are designed to match standard 5mm or 5.08mm spacing terminal blocks. We do have some of these 3-pin terminal blocks in stock (sourced from China so I don’t have US part number). You can send an email to [email protected] to ask about it.
I have no experience with python, but I have coded in BASIC, C, and PERL.
The interval_program is written in Python, the sprinklers_pi program is written with wiringPi (which is similar to Arduino, a Java-based language). There is one very simple demo (selftest) written in C. You can use that as a starting point to see how to activate valves.