Your plan includes a set number of template slots. Slots are counted in canvases across your whole team: a single-format template uses one slot, a multi-format template uses one slot per format. Templates in the Drafts, Archive, and Presets folders do not count, so you can park work in progress or retired templates there without using a slot.
When you reach the limit, the Create Template button is disabled until you delete or archive a template, add template slots (Pro, under Settings → Billing), or move to a larger plan.
Your plan also includes a set number of stored assets in the asset library (images, videos, documents, and fonts; files linked to a campaign count too), managed the same way. You can see how many slots your plan includes and how many you have used under Settings → Billing. The allowance for each plan and the price of additional slots are on the pricing page.