I trying to make concrete counters and am told that I should use either a "6-bag" (or "7-bag" mix). What is the ratio of cement/sand/agg. for this mix?

By volume, you can use a 1:2:3 ratio of cement, fine aggregate (washed sand) and coarse aggregate, probably a pea gravel in your case, should get you a 6-7 sack equivalent.