Manuscript question.

You don't need to mention it. They'll assume others are reading. The only time you need to tell one agent that another is reading is if you get an offer of representation. Then you notify all of the agents currently reading to let them know.

Cyia has the right of it. I sent my first subs out in a batch of 6, which i think is reasonable number to deal with/to figure out if your query letter and pitch is working.

Sometimes they will ask if anybody is reading it, or want a period of time to read it exclusively. Only then do you need to advise them.

My advice is not to say anything, though it is tempting to let them know somebody else also has the full. Now, if they ask you whether another agent is reading the full, that's different. After seeking advice from AW gurus (see recent post in this forum called 'Why do Agents ask these questions') I decided from now on to tell agents who ask if others are already reading my full. And name them if requested. But again, only if they ask.
HTH