The maximum possible value was estimated by taking the sum of all the S/C Values in the wiki, and then adding to that 2.34 (found by adding up the learning ability %'s and then subtracting 1) times the value of the best in game curiosity for S/C currently multiplied by the number of unique curiosities in game. At the time of this post, there were 81 unique inspirational reported on the wiki. This would consume 1134 inspirationals, 1053 of which would be beaming proud sunflowers. All purities were assumed to be 1.00 for this particular test.
The maximum level of stocks and cultivars obtainable at 1.00 purity is 6467 using this math. If it were possible to get purity 10.00 beaming proud sunflowers (which it is not as of current) and all other curios were 1.00, the maximum possible value is 61171 levels of stocks and cultivars.
Edit: If you had lvl 6467 S/C, and got the full plenty bonus from planting. The field would generate 131.34 ears of cereals (rounded up to 132 the way the game does it). This would fill 5.5 inventory+backpacks, and would be enough seeds if threshed to replant 262.68 fields at 50% upkeep.