Auto numbering Google sheets

I want to have auto numbering dependent on cell drop down box in helper column so i can sorted out.

If someone pick Start in A2 to have number 1 in B2.
Then if they skip one or two and chose StartA4 in B3=2,
A6 in B6=3
A3 in B3=4
Then auto sort the sheet based on B column?

https://docs.google.com/spreadsheets/d/1hu-NQ3QpeP-_SMwiSm3EY4uyVS36cswYrInOwgy7oP0/edit?usp=sharing