How much teaching is expected on a CV for a graduating PhD student (mathematics)? In previous comments/discussion (which I cannot find at this moment) a few people from this site noted that PhD students (in math/CS) should make sure they acquire a good amount of teaching experience before they graduate, and that this is something people look for in postdoc applications. In your experience, how much teaching experience is necessary, and what's a good amount?

I ask because after obtaining fellowships I have not been able to obtain TA positions because there are a limited number and they are given first to students in need of support (perfectly reasonable). Thus I have TA'd a total of 3 quarters: three upper division courses where I had to put together quizzes, grade homeworks, and lecture twice a week. It is likely that I may graduate before receiving another teaching positions. Is this okay, or should I really be looking more teaching assignments? 

Note I am trying to supplement this lack of teaching by teaching workshops on things like programming in Julia, big data analysis, and machine learning which are run as part of an on-campus interdisciplinary center.
It is what it is.  Some schools give you less opportunity for teaching than others (e.g., schools in the UK).  

If you are looking for a research postdoc, then having a lot of teaching experience is not too important.  For instance, at my institution, we want to make sure you're competent and not going to cause problems, but you don't need to have won five teaching awards.  This is evidenced primarily by your teaching letter.  If you think you can get a decent teaching letter now, then I wouldn't worry about it.  (Note: if you're TAing now and probably won't be again before you apply, consider having someone who will write you a letter observe your class.)

If you're looking for a "teaching postdoc," e.g. a temporary position at a liberal arts school, they typically want more evidence that you're a good teacher, so you might try to actively look for more teaching opportunities.

Note: presumably many other students at your institution have the same issue.  Try finding out how much TAing recent graduates did and what kind of jobs they got after graduating.