hard to say as I really never had to reseal an entire car.
I would say that a qt would be fine for the floor and maybe part or all of the trunk.it just depends on how thick you brush it on.

How to apply is a no brainer it's called brushable seam sealer for a reason
if you want it to look smoother you can take some thinner on your finger after you brush it on and smooth it out.
I tend to use the seam sealer in the caulk tube as it is easier to dispense then having to scoop the stuff out of a can with a brush,but it takes several tubes to do a whole floor.the qt brushable might be more economical as it will go further for less money.
another trick to keep the seams looking neat is to tape 3/4 masking tape to either side of the seam then brush on your seam sealer then before it starts to set remove the tape and it will give you even edges and you can do he thinner trick after you remove the tape to smooth the edges