Calling in consecutive days counts as a single occurrence... that is, calling in two days in a row is the same as calling in one day. You can call in more days in a row, if need be, but you need a doctor's note on the third (or more) day.
As for how many you are "allowed," while someone else outlined the "official" company stance, it really depends on what your job is, how desperate they are for people, and what the flavor of the month is. If you are pt, you can call out all of the time, and stilll keep your job. I've never seen anyone who was out of their probationary period (in my center, at least) be permanently fired for attendance. If you were in package car or feeders, I imagine they would be a whole lot more strict about it.