Tim, I knew you could have more than one, but I thought it had to close proximity of the
meter or sub panel. According to NEC it just needs to be at least 6' apart. The house we sold had three in a triangle, bonded, where the meter and sub panels were. But on the other side of the house was another one off the basement. That one was the culprit for me. I was told it had to hooked to the main ground system.
Wow, no grounds at all, and California no less... 
When I hire a inspector I find things I usually fix any way that was missed. When I sell, the other persons realtor always gets someone that finds something I have to fix as well.
When my son bought a house here in Indiana, his inspector hired by his realtor agent, missed a big one. I wasn't there. But a couple years down the road, I've been helping him do some remodel. When I asked him where is sub panels were, he pointed, picture me banging my head on wall, they were in the master bathroom 3 foot from shower, in a closet, not separated, just a small utility closet. So we're going to move them before we put the bathroom back together. 
Of course as long as you own the home, everything is fine! Until you try to sell, at least for me.
