Do I still need my own HST number if the marketplace I sell through already collects tax for buyers?

Possibly, and the platform collecting tax at checkout doesn't automatically mean you don't. Your own obligation to register for GST/HST generally depends on your own sales exceeding the small-supplier threshold, currently $30,000 in worldwide taxable supplies over the relevant period, regardless of whether a marketplace is separately collecting tax from buyers in situations the platform rules make it responsible for.

Where the platform is the one legally required to collect tax, commonly for non-resident vendors or specific facilitated categories, that's a separate obligation from your own registration requirement as the underlying seller; being below the platform's own collection trigger for a given transaction doesn't mean you're below your own small-supplier threshold, and vice versa. A growing seller can cross the small-supplier threshold and need to register and charge their own HST even while continuing to sell through the same marketplace.

Because these are genuinely two different questions, what the platform is required to do, and what you're required to do as the seller, track your own sales against the small-supplier threshold directly, and don't assume the platform's checkout tax handling means your own registration question is settled.