I'm confused by the answer Dave gave ("you can't add  1 to an
alphanumeric string"). This is indeed true (types are mismatched),
but the Subject Line refers to AutoFill. AutoFill can do this..so...

Starting with this:
A1 = AB001
A2 = AB002

This recorded macro:
Sub Auto_Fill()
Range("A1:A2").Select
Selection.AutoFill Destination:=Range("A1:A9"), Type:=xlFillDefault
Range("A1:A9").Select
End Sub

Gives this:
A1 = AB001
A2 = AB002
A3 = AB003
A4 = AB004
A5 = AB005
A6 = AB006
A7 = AB007
A8 = AB008

I do not, however, understand the question about the limit.