What does a No. 1 single fetch a singer these days? If you're Lisa Loeb and the single is ''Stay'' the surprise hit off the Reality Bitessoundtrack it's an all-out bidding war. Once Loeb's song began heating up the charts last spring, several major-label execs outdid themselves trying to sign her. ''It was funny going to (Lisa's) shows and seeing all these record-company people there,'' says a spokesman for Geffen Records, which will soon announce it has signed the 26-year-old singer. ''Everyone was looking at each other, thinking, 'I wonder how well they're going to do?''' In the heat of battle, other labels, including RCA and Madonna's Maverick, lavished the star-to-be with gifts (like clothes and CDs), but the Geffen deal gives her the chance to share a label with the likes of Courtney Loveand Sonic Youth. Loeb is currently holed up in upstate New York, whittling down a repertoire of 30-plus tunes (representing ''seven years of writing,'' says her producer) to a more releasable 10 songs, which may reveal a deeper, darker side to the sweet, wispy-voiced chanteuse. Says a source close to Loeb: '''Stay' is as sweet as we're going to get.''