Welcome!
The backtest number is the last part of the backtest URL. Run a backtest. Look at the browser URL. It should look something like
https://www.quantopian.com/algorithms/589caef4fe39847b06a42e4e/589cbe89c7f02a5e0e8e5bb0
or
https://www.quantopian.com/algorithms/589caef4fe39847b06a42e4e/589cbe89c7f02a5e0e8e5bb0#backtest
The backtest number is the number after the last forward-slash (exclude the '#backtest') In this case it is
589cbe89c7f02a5e0e8e5bb0
Paste that into the "number" field in the get_backtest("number") and you should be good to go.