Suggestion

A feature to limit agent usage on a per project/plan basis. E.g.:

• Project A cannot use more than 5 agents at a time
• Plan B cannot use more than 6 agents at a time

Since a plan can have multiple jobs, and each can run on a different agent, there can be scenarios where a given plan is using too many agents, causing other builds to wait. 

Workaround

Add a custom requirement to each job from the plan, and have that capability only available on a specific set of agents, so only those can be used to run the jobs.
Configuring a job's requirements