It's 2 per 500pts. Which is not very limited at all.
With 4AC it would become the carrier of choice, beating out all other options. Why buy a dictator when you could get the same AC for 80pts less? It would also allow AC spam on a scale the IN has never before been capable of. 6 Defiants, an Emperor and a Dictator for under 1500pts, or 36AC, an increase to 150% of the current maximum at those points.
Even with a more severe limitation, 1 per 500pts, it would make no sense to take any other carrier until you'd maxed out on Defiants. You'd have broken the ship in the opposite direction. This is why it could never be AC4.
Now Admiral d'A would like to give it S3 forward facing Lances or S6 Torps, armour5+ and 25cm speed - that's not a Voss, that's a Carrier Dauntless. I'm not averse to such a thing. I'll even propose one:
Furious Class Light Cruiser 130pts
Cruiser 6
Speed25
Turns 90'
Armour5+
Shields 1
Turrets 1
Port/SB LBs S2 Total
Prow Lance S3 30cm F
Special Rules:
+D6 AAF
Prow Lance may be exchanged for S6 Torps.
There. Now that's out of the way, we can return to what to do with the Voss pattern carrier. It has neither the speed, range or power to strike from afar, and will do best hidden amongst the ships of the line, mainly providing CAP, as its bombers are otherwise only good for attacking cripples.
In this role it lacks two things: firepower and incentive to reload. It is not more important to reload 2AC than it is to lock on your 45Gun Battleship or to CTNH with an entire escort squadron. Giving the Defiant Torps will help with this a little. As for other weaponry, after the torps the Defiant would be at max capacity with 1 prow lance or 3 WB, which isn't enough, so it requires dorsal weaponry as well. We want to add to its weight of broadside and not to overpower the prow, so L/R arc would be sufficient.
The new Defiant would therefore have weaponry in the range:
Prow:
2-3WB or 1 Lance 30cm F/L/R
Dorsal:
2-3WB or 1 Lance 30cm F/L/R
2-4WB 30cm L/R
Personally, I don't think it would be overpowered picking the upper end of those ranges, but Horizon's profile has conservatively gone low end.