But some are saying that you have to wait for letter of completion after that you can work full time till the next course starts… but if I wait for the completion letter .. then only I left with 1 month August, all my these 3 months got waste as per 20 hours work limit, so is that true or not ? Please help me out in this
I'm assuming you are talking about 2 different programs which you are doing - 1 you have completed and 2nd you have been accepted in to and enrolled. If this is the case, in theory you have to leave Canada and reapply for a new study permit as you don't have status after you have graduated unless you have applied for a PGWP.
Applying for a study permit extension puts you in implied status under which you maintain your status at least until a decision is made. This allows you to work full time.
Now, if you are referring to same program and your next course for the "same program" is in Fall, you don't need to do anything and are allowed to work full time until start of school year. You will however have to apply for a student permit extension if you need more time to finish your degree/diploma/certificate.