I usually try for 10 salvos, and usually manage around 1 launcher per 1000 tons of (dedicated) missile ship. But more of both is always better. And of course, you also want your missiles to be as fast as possible. The way I think of missile point defense is in terms of "speed capacity."
# of shots * tracking speed * accuracy/incoming missile speed is the expected # of kills you get, so a point defense setup (which has fixed # of shots, accuracy, and tracking speed) can be evaluated in terms of its speed capacity.
ex. the Amun-Ra has 8 shots at 10,000km/s, so its PD suite has an 80,000km/s capacity. On average, it'll stop 4x20,000km/s missiles...or 1x80,000 km/s missile. Point-blank range for regular weapons is technically not 100% accuracy, but crew grade and officer bonuses can usually make up the difference, so I tend not to account for that.
How is this relevant to your question about # of launchers? Well, you can do the analysis in reverse to figure out how hard a salvo will be to stop. Figure out what you need to penetrate, then work backwards. Usually you don't have the luxury of knowing opponent PD capacity BEFORE designing your ships, but you can use this to get a sense of how well various missile suites compare with each other.
Also, reduced reload rate (and Box launchers) can really help out here. My current game has 15,000 ton missile cruisers using reduced reload rate size 6 launchers to get 12 missiles per salvo despite heavy armor, PD, and an oversized propulsion suite. But I noticed that, if I were to build a 20,000 ton carrier and load it with box launcher fighters, I can achieve a 55 missile salvo. All told, the carrier + fighters would cost about twice as much as the cruiser, and would have lower strategic mobility because the carrier is slower than the cruiser...but its damage output would be very competitive.
