There can only be one new topic draft at a given time. There can be multiple new reply drafts though.
As a staff member, to get around the issue of only being able to have one new topic draft, you could try using shared drafts. The intention behind shared drafts is to allow multiple staff members to work on a topic before it is published, but shared drafts can also be used by an individual staff member to create a topic over a long period of time.
When a shared draft is published, its timestamp will be updated and the edit history of its first post will be removed.