How much: Enough to prevent any water from getting inside any holes or gaps, and enough to satisfy your cosmetic desires of how the finished seam looks.
Where: Everywhere there is a potential hole or gap (along the entire welded seam), or everywhere you want to cosmetically camouflage/hide a weld.
Brand: Meaningless in my mind, buy any brand you want that is compatible with whatever refinish process you plan to use. Some are runnier and more difficult to use. Some cure faster and some slower. I usually prefer to look for a heavier bodied seamsealer for ease of use, especially if working overhead.
I personally usually like to run a bead along the seam, and immediately spread it lightly to work it into the seam with a disposable acid brush or similar, to give it a uniform brushed look. Others who want cosmetic perfection in seamsealer will often mask either sides of the desired seamsealer bead (maybe approx 1/2" to 1" wide gap?), then apply/brush it on or smooth it to their satisfaction, and immediately pull the tape off before it cures to get beautiful/clean/crisp lines along the seam, for cosmetic perfection of a symmetrical and straight seam. You can also choose to wipe with a little thinner before curing if you want to smooth it perfectly instead of brushing and leaving it with a brushed look like I usually do, especially on a floor or hidden area like you're talking about.
Basically I feel there aren't many rules here, unless you're trying to do concourse reproduction of what the factory did, in which case I am of no value because I'm much more of a hot-rodder than a restorer...
Want other ideas, Google "how to apply seam sealer" and read 100 other opinions, and even see some Youtube videos and photos.
