And here is what I googled to get

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.