The function of alchemist's supplies and herbalist's kits are a little vague and scattered in the rules-as-written of 5e D&D. Based on my understanding of the rules and what I think they're intended to do, this is how I'm implementing their usage in my games.
What does proficiency with alchemist's supplies and herbalist's kit let you do?
Proficiency with the herbalist's kit lets you craft potions as per the crafting magic items rules in the DMG, and it lets you craft potions of healing and other consumable items that might count as "remedies." The cost of the materials needed to craft a standard potion of healing is 25 gp, and takes 10 days of work to create. If a potion explicitly replicates a spell, you need to be able to cast that spell as part of the crafting process. This kit also allows you to craft antitoxin, and identify herbs and botanical ingredients.
Essentially: I'm treating garden-variety potions of healing as mundane items, as otherwise their price in the PHB doesn't make any sense. Other potions, however, will be crafting using the rules for crafting magical items as per the DMG.
Additionally: Although this isn't covered by the rules, it makes sense that you could create more than one potion of a particular kind at a time, so I'll definitely allow brewing a "batch" of potions if you have the time and money for the ingredients.
Alchemist's supplies and herbalist's kit descriptions: PHB 155
Crafting downtime action: PHB 187
Crafting magic items: DMG 128-129