Yes, it is best to take the Subject Test right around the time you finish the class. Two is typically the most required (unless you apply to Georgetown - they require three). However, I know plenty of kids who took more than two or three, especially if they knew they could do well.
Even back in the dark ages (back when they were Achievement Tests) I took four tests - I don't even know if they were required back then. Kids around here have always taken Subject Tests. Perhaps in some areas they aren't as popular, so it seems like a big deal.
I can certainly understand why these colleges require more of homeschoolers. I know a homeschooler that sent a kid to West Point and I know a homeschooler that had an eight year old who could not read. There really isn't any way for the college to know which kid is applying - you need standard measures so you can compare.
