It's tough to give an exact number. Different places have different amounts. Urban centers tend to have more, but it really varies by location and the popularity of comics in that particular area.
It's possible for comic shops to serve as CGC witnesses. However, they need to meet certain criteria set by CGC, such as having appropriate expertise and adhering to specific protocols.
It's hard to say for sure. There probably weren't a huge number, but exact figures would be difficult to obtain without comprehensive industry data from that time.