Hi Kirk
That's kind of a trick question. Normally I use a couple coats, sometimes three. The first thinned out. Remember, there not really coats because each layer melts into the last. And if you're brushing shellac I like to coat once in the morning and a second before I leave the shop at night. Coats to close together can wrinkle up. If that happens, walk away, let it dry, and it should level itself out.
So here's the tricky part, lets say your using black walnut and it has a strip of sap wood in it. With shellac you can add color to the shellac and brush, pad for better control, the area to correct the color. With most finishes you need to have the color right before you start to finish, but with shellac color can be added anytime, through out the entire finishing process. If the area is small, using dry powders, dip your finger tip in the powder, wet the surface lightly with a pad, and add color to the surface with your finger. Then lightly hit the area again to lock it in. You're now French Patching. And if the color is not right, simply add a little alcohol to the pad and erase and try again. With walnuts,mahogany's, and other dark woods I start by using an orange, sometimes red powders and build on that. Reading a little color theory comes in handy at this point. Before adding a different top coat I like to scratch the surface with 4/0 steel wool to create a mechanical bond rather than just a chemical bond. With practice, patina can become a process rather than a reaction of time. This can get a lot more technical but this is a good place to start.
You don't know if you can do something until you try.
Regards, Randy