I cannot add another agent
Agents are counted against your plan, the same as numbers.
Every plan includes as many agents as numbers — 1, 3, 5 — because a number routes to exactly one agent.
Free a slot
Archiving an agent releases its slot, and nothing about it is lost. You can restore it later if there is a free slot.
Or take a larger plan
A larger plan also raises your plan minutes and how many calls you can take at once. See Plans and minutes.