Bearing in mind the comment by cvg, we have designed walls in excess of 30 feet. Depending on the equivalent fluid pressure and slope of the backfill, calcs usually have the first 4 to 6 feet of CMU, 2 to 6 feet of 12 inch CMU, and then 2 to 6 feet of 16 CMU and then concrete from there. Again, there are many factors this is the typical sequence up to 12 to 18 feet.
There are many inexpensive retaining wall programs on the web, including retainpro that provide the necessary design info and calc quickly and efficiently.
Good luck.