I'm sending five.
Five at a time is apparently whack-a-doodle crazy numbers and I'll "never" get an agent that way. That's what I've been told by an anonymous agented third party (who I'll allow to remain anonymous for the sake of their um, anonymity).
I'm not sure when querying became about simply getting an agent, any agent. That doesn't fit with my (possibly misguided) view of how things should work. Querying, to me, is about finding the right agent. Finding an agent that fits well with how I work, what sort of stories I want to tell, and most importantly, finding an agent for my writing career.
It means I can't take just any agent. It means I have to
I can't imagine doing it any other way, I think the stress of having more than five queries out at the same time would kill me. If that makes me some sort of oddball, so be it. I'll get an agent when I do. And not just any agent, but the right agent.