There are some trade-offs to consider.
Only one person can check-out and edit a TD plan at one time. So, if you want multiple people -- like team leads -- to help set up and maintain parts of your plan, then you may want multiple TD plans. (It doesn't matter for check-out and editing purposes whether they are in separate projects or not.)
It can be difficult in TD to gather and then view or report status across multiple projects and/or plans. Go to the "Analytics" tab/application to see what is possible in the web interface. Status is at the project level, and it can only be provided by one person at a time. (The project status displayed in the Analytics tab is always the status that was provided last.) So, if you want multiple people to provide a TD project status, then you'll want multiple projects. However, if you need status but don't need multiple "TD project statuses", then you could conceivably have people provide their status through project feed comments or other means and have one person assigned (you?) to consolidate them into one TD project status, e.g. every Monday.
A plan task can have a predecessor that is a task in another plan (that can be in another project). However, it can be difficult to set these up and keep them maintained. So, that's an argument for having one project plan with multiple tasks, rather than disparate plans.