In Jupyter, the number on the left are not the number of the cells but the order in the execution. IPython has the very same behavior
$ sage -ipython
Python 2.7.14 (default, Dec 13 2017, 23:18:38)
Type "copyright", "credits" or "license" for more information.
IPython 5.1.0 -- An enhanced Interactive Python.
? -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help -> Python's own help system.
object? -> Details about 'object', use 'object??' for extra details.
In [1]: 1+1
Out[1]: 2
In [2]: 3+5
Out[2]: 8
In [3]:
But in a console as above, you are not allowed to edit the input lines as you would do with cells in a notebook.
The proper way to reinitialize the execution of the kernel (and hence starting again from 1) is to do Kernel -> Restart.