What should I do if I found too many typos after my Ph.D. thesis being approved but before my oral defense in mathematics?
Talk to your advisor. 

When I defended, my committee came back with a number of such corrections to be made. I made them and they signed off. Then the thesis editing office took the document and came back with even more corrections. I made those, and eventually they signed off. Trust me, you aren't the first to deal with this problem, and likely not the first that your advisor has seen.