If I produce a herbal formula and see if it sells can I later patent the formula later?

you have 12 months to file a patent in the US after you disclose it the public or offer it for sale.

You have one year in the United States to file a patent application on something that you sell, counting from the first sale or public disclosure, before you lost the chance to try and patent it. Outside the United States you do not have any time at all and must file the application before the first sale or public disclosure otherwise you immediately lose the chance to try and patent it.

According to the newly adopted Smith-Leahy patent reform act, in order to patent a new formulation, public disclosure actions taken prior to application filing can make it impossible to obtain a valid US patent on the formulation. These actions will be relevant in determining patent eligibility for any future modified formulations. In order to obtain patent protection, an inventor/entrepreneur should consider filing prior to any sale or disclosure activities. A provisional application may be worth considering. Naturally, there are unique particulars surrounding each situation. You should discuss these with your patent attorney.

You have one year from the date the product is made available to the public (even a very small section of the general public) to file for a patent.

Maybe yes. To be patentable, your herbal formula must be (1) totally new (never made before); (2) useful (do something) and (3) not be an "obvious" formulation based on what was done before in this area. You can sell or otherwise disclose and the within the one year "grace period" - still file a US Patent Application on the formula or modified versions thereof. If you miss the grace period - you cannot patent your formula.