Now I take the Visa one year multiple and it ends at 15 may 2020 , if I entered Canada for example at April 2020 , so I can stay for 6 months as the standard (so till October 2020 ) or I should leave Canada max. By 15 may 2020 ?
It's a test to see if you will actually comply with the conditions of your TRV. Don't be surprised if CBSA doesn't give you the full 6 months at the time you enter. If you meet the conditions of your TRV and leave within a reasonable time, when you apply for a new TRV, you may actually get a longer duration. The time you get to remain in Canada is determined by CBSA when you enter. May be 6 months, maybe a week, maybe refused at the POE. It's up to them.