Stability of capsules with folic acid, are there stabilised studies available?

There is little information to be found, which can serve as a direct answer to your questions. You can prepare about 10 capaules per turn, taking into account the TMF. Folic acid is mainly subject to photolysis. In the case of trituring, the following rule is applicable within the TMF Committee  

The shelf life for a trituration, retained under the conditions prescribed by the TMF, corresponds to the shortest retention period of the raw material, which appears in the composition of the dilution. In principle, a dilution can be stored for more than 2 months.     As far as stability data are concerned, I could only find the following

Folic acid is unstable and loses its Activity in the presence of light, oxidizing or reducing agents, and acidic and alkaline environments. However, it is relatively stable to heat and humidity; Thus, premixes, baked products, and cereal flours, retain almost 100 percent of the added folic acid after six months of storage. About 70 percent of folic acid added to wheat flour is retained during bread baking.