I am assuming you will have a car to reach your accommodation and to move around and that you're interested in taxi service for evenings when you might want to dine and drink at meals so that you don't have to worry about getting back to your place.
Unfortunately, there is no set taxi service in the smaller towns across Tuscany, they depend on local drivers where you book their service ahead of time and, especially in the high season, you can't depend on finding one to book on "the spur of the moment". Maybe in low season you could, or at the very least, with just a few days notice.
In any case, the winery where you are staying should have contacts for local drivers to the area. Contact the winery and ask them if there are drivers available in the area to act as taxis, or if they all need to be prebooked. If they can share those contacts now BEFORE your arrival, you could already arrange a few outings - or at the very least contact them to find out their rates and what the procedure is for booking their services. You never know, the winery itself might offer a taxi service to their guests!