How much caulking do we need for a standard bathtub
One standard caulk tube will do a tub plus another.

https://www.homedepot.com/p/GE-10-1-oz-Clear-Silicone-2-Kitchen-and-Bath-Sealant-GE5040-12C/100663319

plus

https://www.homedepot.com/p/HDX-smooth-rod-caulk-gun-10-oz-HD109/203788465

The caulk link above - my go to.   The gun is just an example (that brand gun is actually crap and pushes out too much even when no pressure).

The way I caulk a tub is to run a first pass that goes as deep in as possible, hit it with a wet paper towel or sponge (push the caulk in), wait about a half hour, and then do the second coat and smooth the finish.