1 fort per planet per type would be too little in my opinion. 2 per (like mini-forts) would be fine with a 1 per for mk 3s, and the energy cost is already extremely prohibitive to even think of doing that.
In our 10/10 game I was desperate to have forts everywhere I could and I actually ran out of energy and couldn't place them all with the normal 4/3/2 cap, so if I can't even place the normal cap around the galaxy when I'm not doing crazy 'let's not care about AIP' maps I think it will only hurt people playing on diff 10 (probably 9 too) to be stuck at only 1 fort of each type on a planet.
I do like the idea of per-planet defenses, quite a bit. It's why mini-forts are always unlocked immediately just like metal/crystal harvesters. Especially in multiplayer just having a minimum amount of defenses at every planet at the start is better than the extra fleet I could have got.