* "The agenda is attached" is correct if there is only one agenda attached.
* "The agendas are attached" is correct if there are multiple agendas attached.
You can also use "The agenda is attached" even if there are multiple agendas, as long as you are referring to them as a single unit.
For example:
* "The agenda is attached for tomorrow's meeting." (Even if there are multiple items on the agenda)
* "The agendas for each department are attached."