Does my corporation need a new HST number after it amalgamates with another Ontario company?

Not necessarily a brand-new registration from scratch, but amalgamation does require sorting out which GST/HST account the combined corporation will use going forward, rather than assuming everything continues automatically exactly as before. On an amalgamation, the resulting corporation is generally treated, for many purposes, as a continuation of the predecessor corporations rather than as an entirely new taxpayer appearing out of nowhere, but the practical GST/HST registration details still need attention, since the amalgamated entity typically needs to confirm with the CRA which existing account, if any, it will continue using, and close out any accounts that are no longer needed.

Where the predecessor corporations both had their own separate HST registrations, simply continuing to file under both, or under whichever one happens to be convenient, without formally sorting this out with the CRA can create mismatches between the corporation's legal existence and its tax filings. This is exactly the kind of administrative detail that's easy to overlook amid the broader legal and corporate work involved in an amalgamation.

Because the specifics can depend on how the amalgamation is structured and which predecessor accounts existed, confirm the correct registration approach with the CRA and your accountant as part of the amalgamation closing steps, rather than assuming the old registrations simply carry on unchanged.