In my experience any salvo lower than 10 missiles is doomed. Taking into consideration the FC/launcher size ratio means I never have more than 1 FC for every 10 launchers - and most of the time the extra FCs are lower range since by the time I probed the enemy's defenses they are most probably quite deep inside my firing envelope.
So far the best way I found to overwhelm any point and interceptor defense is with a mix of missile platforms ( 500-1kt , no engines , 1 tractor beam and a lot of box launchers) and ship-board boxes for the first salvo , with time-on-target attacks from the rest of the dedicated missile ships.
What does that mean ? with some modest tech and not that many ships (2-3 platform carriers , 3-4 missile ships , boxes on the non-combatants) you can deliver up to 500 or more size 5 missiles. I have not encountered any missile defense that could counter it coming from the ai - and it wiped half a fleet when playing a multi-empire setup.
Adding 1 unit of armor to those missiles will make them even more effective - there's just no way the enemy can launch enough AMMs to counter all of them.
You can consider sending a salvo or two of heavy armored big missiles 5 sec ahead - if you use ecm on the rest of the salvos it will drain AMM stocks , generate lots of missed interdictions and , if you are lucky , distract the final defensive fire enough for the massive cloud of death behind to hit
Now on the intercept side :
- AMMs CAN have 50% chance against other missiles once you reach inertial fusion and l4 agility/warhead. And so far it seems a faster AMM is more efficient than a agile one since you can start intercepting at a higher range - which means your launchers will not concentrate on a missile at 10s while there are a few billion more just behind.
- It's better to have your AMM ships behind the fleet , using small platforms for them ( corvettes/frigs , up to 4k). If you also put a PD element in front of the big bathtubs in your fleet (about as far as twice their effective range) you can also aid the AMM FCs and reduce the hits.