I'm applying even to less -- either two or three. This is my first application season, and honestly, if I don't get into schools I really want, I might try law school. It's not that I think I won't learn a lot from other schools or that it won't be a generally rewarding experience. But my other love is law school, I have a good chance getting into a really good one (maybe not YLS but very possibly Columbia, Chicago or Stanford), and I think that I'd like that more than going into a school that was my second choice.
My interests are actually very similar to OP: Rabbinic literature in its late ancient context and secondarily Christianity in the Roman East and the Sasanian Empire. My top schools are Princeton and Yale, and I'll definitely be applying to them; I'm also thinking of possibly applying to Harvard or Columbia.