The only limiting factor is the morphs genetic locus. Long genetic lesson made short. A morph is caused by a mutation in a gene. Some morphs are caused by mutations that are on the same gene as another morph. For instance you can't get a lesser mojave because instead you will get a blue eyed leucistic and that BEL could never pass mojave and lesser, its one or the other. Its important to remember that we are simplifying a very very complicated subject. There is potential for countless mutations. Of those mutations a subset will lead to healthy adults. A subset of those will lead to some outwardly visible difference that we would call a morph. Those mutations could be combined with any other mutation that is not on the same gene. Not sure how many genes BP's have but we can roughly compare them to humans and we have 20,000 to 25,000 genes not including mitochondrial DNA.

So to sum up and answer....the limits of genetics are way beyond what we as humans have been able to express so far. The real limit is on how many morphs we can find in nature and the time and effort we are willing to spend adding them together in one animal.