Is Chell the last person alive?

In Portal 2, Chell awakens and hears that 9999... years have passed. The surface is just a wide field. Is she the last person on earth?

Solution 1:

Is she the last person alive...

No. There are plenty of additional test-subjects that are uncovered at the end of the co-op campaign

on [the surface of the] earth?

Nobody really knows. Killing everyone on the planet is quite difficult, and the ending of the Half-Life series (as the two exist in the same universe, but vastly separated in time when Portal 2 occurs) would probably provide the best answer. I would assume that the Good Guys win, so people will still be around, but who knows how many or where they're at?

Solution 2:

Yes there ARE people left on the surface. At the end of Portal 2 you leave the elevator and are confronted with fields of wheat. Wheat fealds are quickly over run by weeds when left unattended. As there isn't a single weed in the fields then there must be people left alive to tend to them......

Solution 3:

GLaDOS also mentions at one point she's been to the surface and seen lots of humans - but won't need them as she has Chell.