There are 2 practical limits that I know about. First, the more cream, the more difficult it is to get rennet to set the curd. This is much less important in a semi-lactic triple creme. I'm not sure what you mean by "Triple creme Camembert" because Camembert is a specific style of cheese that does not have added cream. It has normal amount of rennet (say ~40 IMCU per liter), cut and drained. Triple creme style cheeses are sometimes (often?) semi-lactic or full lactic (at most about 10 IMCU per liter of rennet and sometimes no rennet). I assume by "Camembert" you simply mean that it has penicillium candidum growing on the outside? If you make a triple creme using a Camembert style make, the curt won't set well and it will likely lose most of the fat when it's draining. For a semi-lactic/lactic (small amount or no rennet and waiting 8-12 hours before draining), you can go pretty high. I've heard 80% fat by dry weight is achievable. However, the more you add, the harder it is to drain, so it might take you a week or even more for it to drain to the right consistency. Boursault is apparently 75% fat by dry weight, so you should have no trouble reaching that. In terms of a recipe...
My last (actually not quite) triple creme using something similar to a Camembert style make was 2 liters of 3.8% fat and 3.8% protein and 200 ml of 47% cream. My bad math tells me that's just under 70% fat by dry weight (2000 x 0.038 + 200 x 0.47 = 170 grams of fat, 2000 x 0.38 = 76 grams of protein, 76 + 170 + ? (ash content) = > 246 grams total solids, 170 / 246 = < 69% fat by dry weight -- technically triple creme is > 70% fat by dry weight). To get this to set reasonably I needed 20% extra rennet and even then I got a 22 minute flocculation time.
So, for my milk, to get up to 75% fat by dry weight I need at least 330 ml of 47% cream. At that point I would not use a Camembert style make. I haven't tried it, but I don't think it will set well at all. I would go with a semi-lactic (probably about 15 IMCU per liter -- 1.5 drops of single strength rennet per liter of milk) and a 8-12 hour set time, followed by a 12 hour drain, flip 12 hour drain, flip then flip every 6 hours or so until it can stand freely (probably several days later). I've never done it with this much cream before, though... So this is just my being theoretical.
Some triple cream producers add extra protein to the mix (using a kind of Frankenstein milk). You can do that by adding skim milk powder (must be freeze dried milk powder, not heat dried!). I've done this before and it works surprisingly well. However, you have to do the math to figure out the ratios to hit 47% cream. The idea of using dried milk powder is that you get the extra protein without the water. This makes it set more easily and it will drain *much* more easily. So if you get stuck, this is a possibility. Normally, with my bad math I just assume skim milk powder is 100% casein, but this is completely false :-) Probably only about 80% of it is...
Hope that helps you and doesn't scare you with the crazy math.
