It seemed that on this site, people were in agreement that the Yankees could only sign three Type A FAs. However, I'm not so sure.
According to this, you can sign as many type A/B as you lose. Also, this says that the Yankees have four type "A"s (Damaso Marte - really?) and one type B (Pudge). The Yankees have already signed Marte, so they might only lose 3 type "A"s.
So I guess the questions are 1) are teams capped at 3 FA signings. That doesn't seem to be the case since the Yankees have already signed Marte (who is, again, amazingly a type A); 2) can the Yankees sign 4 type "A"s and 1 type B (in which case they're done on type "A"s); or 3) is the type A/B thing an aggregate, so the Yankees can sign one more type A (Manny/Sheets).