If you mix 6 spices into the blend, you will be able to choose three benefits. If you mix 10, four benefits and so on. Currently, the maximum you can put in one blend is 18, using the Pungent Spices feat, so it is impossible at the moment to get all six benefits from that ability. You will be able to get five by putting 15 spices into it, however.
And yeah, the Constitution cap will limit that. It's going to be a hard optimizing work to get a +18 modifier, that's for sure. I know I did it while playtesting and trying to break the class, and the result was that putting all points into that blend makes you quite useless. Invulnerable, but useless. Therefore, I believe it's fine that way!
When we get down to write more, we will add items that give more spices, possibly a feat FoI-style, maybe a stoner-tuned race, and we were thinking about a prestige class or two as well. Those should make it possible to reach 21. In the meanwhile, nothing stops you from making your own magic items or asking the DM for some.