If your gross self-employment earnings (total sales) worldwide exceed $30,000, Canadian law requires you to register, charge and remit GST/HST (a few exceptions do apply).
If you are running a short-term residential operation (less than 1-month stays, such as AirBnB) then you are required to charge HST if the revenues are over $30,000. If you already have an HST number, say for an existing business, you must charge HST on short-term rentals, even if the revenues are less than $30,000.
If you report over $30,000 of self-employment earnings (before expenses) and do not register, CRA may assign you a business number retroactively, and you will be responsible to pay back GST/HST taxes to the time the number was activated.