I'm usually juggling 2 - 4 PBs at a time. One or two at more of a "polishing stage" either sending to critique groups to get feedback or my agent to get her feedback. One where I've got the sh***y first few drafts done and I'm trying to make it more solid, and one or two ideas where I'm still researching (I do a lot of NF) or just getting the bones down.
Like Debbie, I like to let things rest. When I'm tired of one, I just jump to another. Or if one "calls" me more than others. When I wasn't agented, I definitely always wanted one or two "in circulation" with agents (or the occasional editor.) Now that I have an agent, sometimes I have to work on something she wants vs. what I want, but that's OK, too.
And yes, query one PB to an agent at a time, but when I was querying agents and had several polished stories, I sent out the story that seemed the best fit for that agent. So I actually queried 2 or 3 PBs at a time, but not all to the same agent until they asked, if that makes sense. A spreadsheet was the only way for me to keep track! :-)