The number of E-Matches you can connect to a firing circuit is a question with several parts. First, consider wiring in parallel and in series. Both have pros and cons. The most common failure in wiring in parallel is that it requires the most current from your firing system, and you are unable to correctly test continuity. If you need or strongly desire to wire in parallel, do so with only two matches at a time. You can use other firing channels at exactly the same time to accomplish more in parallel, the firing module does the rest of the work for you. The reason for this is that each firing channel produces a maximum of just over 1 Amp. It takes 0.5A to guarantee the match will ignite, so only two.
In series, you can wire up to seven and still have over 1A of current going through the match. More importantly, you can test continuity through all of the matches. The problem with series wiring is if you use a mix of different e-matches, one might fire before the others so not all may ignite. The simplest solution is, do NOT mix matches in a single line. If you have long wires running to the e-match, it will take away some of that current. For example, a 20ga solid copper wire has 10 ohms per 1000 feet (.01 per foot). 22ga has 16 ohms, 24ga has 25 ohms. An e-match is often about 1.5 Ohms (you can test your e-matches with an Ohm Meter to find the exact resistance.) So if you want 1 Ampere of current to light your match (the minimum is 0.5A) you simply add up the resistance, 1.5 ohms per match, and the length of the wire times the gauge resistance. So 100 feet of 20ga is 100 x .01 = 1 ohm. Add the matches and the wire resistance and if it’s under 12.5 ohms you’re good to go.
For example, we had to run a wire out 100m to a line rocket ignition point. If we had extra modules we could have just left one out there, but we had no spares. Several users of other systems did not believe we could light a match at that distance. We were using 20ga wire so 300 feet (close to 100m) is 3 ohms or two matches. So the max number of matches we could dependably shoot is 5 and we only needed one. It fired just fine.