If players don't want to buy fireworks or don't know how to buy them, I doubt increasing the limit is going to make much of a difference.
The limit is not too low. If survivors are getting overwhelmed by commons or the special infected at one area, just buy a Molotov. There are some major advantages to buying this:
1. It saves your ammo from shooting the firework crate
2. It allows you to aim where you're going to throw your Molotov
3. It can be used for not only close ranges, but it can also be used for far distances as well
4. It takes less time to buy and use a Molotov compared to the firework crates
5. It makes you less vulnerable to getting stuck and overwhelmed by commons/special infected when using the Molotov due to Advantage #4
Besides, it's not like the survivors abuse the Molotov.
If anything, increasing the firework limit significantly or making it unlimited again will be like giving the survivors 12.5 points similar to the hunter pounce. They would likely just use them, and abuse them whenever the survivors get attacked by a few commons. You say that limiting the firework limit would give the infected a huge advantage, but increasing the limit or removing it altogether will likely cause the opposite effect.
carols dont qoute my post when you are not reading it clearly, this is an order.