I assume you're talking about distinct job openings, rather than spamming me multiple times about a single opening.
For me, it depends on my current level of interest in changing jobs.
If I'm not looking at all, then I only want to hear about really awesome positions, probably batched into a single daily email. (Daily so that an awesome position doesn't get filled before I can respond.)
If I'm passively looking, probably still once daily, but the filtering criteria are looser.
If I'm actively looking, probably an immediate email for each opening, and with relatively loose matching criteria.
What I never want: being contacted by a recruiter who clearly didn't read what my LinkedIn profile says.