According to the economic census for retail trade (census.gov) in 1997 there were only 126,889 gas stations (64% of them having convienence stores).

I simply took the 1997 and 2002 census data and continued the trend based on the limited data:

1997 - 126,889 gas stations. 81,684 (64%) with convenience stores.

2002 - 121,446 gas stations. 93,691 (77%) with convenience stores.

*2007 - 116,223 gas stations. 104,600 (90%) with convenience stores.

*2008 - 115,223 gas stations. 106,696 (92.6%) with convenience stores.

* Estimated guess based on 1997-2002 trends.