Your business number is necessary for many common business operations, including registering for a GST/HST number with the Canada Revenue Agency. If you have already incorporated, you can find your business number on your Ownr dashboard by clicking the ORGANIZATION tab and scrolling down to "Organization Information."
If you do not see your business number, that means it has not been linked to your Ownr account. For companies incorporated provincially in Ontario, your business number will be mailed to the address of your Registered Head Office 1-2 weeks after incorporating, and you will have to manually add it to your Ownr account. If you are incorporated in any other jurisdiction and do not see your Business Number, you can link it by clicking "Add Business Number" and selecting "Register" from the dropdown menu.