There are some solutions/work-arounds for this, but they are all formula-based and, as such, are vulnerable to rows being moved or deleted.
The simplest work-around is:
1. Add a new column (say C)
2. Set the expression C1: =C0 + 1
Unfortunately, with a solution like this, the numbers won't "stick" to the rows. So if a row gets moved or deleted, then the values will be recalculated and cells will get new numbers.
I hope this helps,