Each time you make a chimera you are creating a new species
Some simple people think that for making a basic sphinx you only need to mix a human and a lion. That somehow you mix both and the result will always be a sphinx. Those that hold such views reveal their uttermost ignorance of the art of chimeracraftship. For it is clear for even the most dumb, that when mixing a person with a lion you could end up with a creature having the head of a lion and a body of a human as much as a one with the head of a human and the body of a lion. Or a lion with an additional pair of human arms, or a person with lion tail, the possibilities are countless. And that is only taking into account the parts of the creatures that will be mixed.
The reality is that the floor lines, the ingredients put around them, and even the chantics themselves are part of the chimeras. They are not some "instructions" for how should the animals mix, but actually become part of them.
Now, it is true that the effect of some ingredients doesn't a big influence on the creature, for instance having a greater number of corn grains will generally just result in a more yellow fur in the resulting sphinx¹ and a proficient summoner is usually able to obtain a decent result even with suboptimal ingredients (like fruit non being fresh), at the cost of a longer process and putting more of its power into the transmutation.
But the fact is that everything taking part on the invocation, from the disposition of the ingredients, the exact intonation, rhythm and pauses used by the summoner, to the original themselves take a crucial role in the forming of the new species. Thus, even though they are all classified as sphinxes, each and every one of them is its own specie, as they have their own lineage (which will only ever have a single individual, sadly).
A few chimera creators have done some experiments trying to create two distinct individuals from the same species, usually by having two summoners build the same type of chimera at the same time, with the same ingredients and using pairs of animals of the same litter. So far with no success.
On the other hand, this is mostly an academic question with most summoners just accepting as a fact that the resulting animals will not be able to breed, and very happy it is this way, given the large profits they make from each chimera they create. Actually their guild would get very angry with anyone that proved otherwise.
Finally, note this is completely consistent that even on the Creation, Women and Men were not created separately but one from another, which for chimeracrafters is evidence it is not possible² to create on different acts two individuals of the same species (ie. two individuals that are able to breed between themselves).
¹ The creation of sphinx by the Ancients show us that they were even able to create sphinxes even without corn, although no corn-free recipe is known today.
² At this point this usually rises a theological discussion on God's omnipotence when there are educated listeners in the room, which leads to their official position stating just that doing so would be extremely complicated and doing things this other way was a much more simple and elegant action from our Creator.