I put a 3rd set of jacks on my last TT. We owned our Springdale (31 foot) for 8 years and I put those jacks on the first week we had it. And to be honest, I can honestly say it did't make a LOT of difference. I think the reason is because the jacks themselves are weak and have a certain amount of flex in them. They wiggle. They help on the up and down movement, but side-to-side, no difference between 4 and 6.
Here's the other problem we had with 6. 4 would snug up to the ground. When attempting to snug up the 5th, one of the other 4 would become unsnug. Add the 6th, and it seem there was always 2 not snugged up. It was very difficult to get all 6 snugged equally. One was always loose.
The only time the trailer sat really firm was when I got a set of bottle jacks (not hydraulic) and put them under the frame. This gave the camper 6 jacks, 4 tires and the tongue jack a foot print on the ground. I did this when parked over the winter. And then, it was difficult getting even pressure on all of them. But the solid jacks (not the scissor style)took care the side-to-side movement out.
For best results, get a set of 4 StrongArm's I've been in 2 different campers that had these, and they make the camper rock-solid.