We don't even KNOW how it's going to sound like.
However, Britney's leads always debut big, even 3, that song's only promotion was a Tweet and it shot to #1 on iTunes and Billboards Hot 100. That being said, #Lead8 will debut with bigger sales than Applause most definitely, but how much bigger will be determined on the material. If it's ace Britney music a la Toxic, Gimme More, HIAM, I'm expecting 300-450k, if it's mediocre, it would still more than Applause's humiliating figure, so around 226-300k.