Blues at level 20 max at 15 light then you need Purples which I believe maxes at 27 And with one exotic which maxes at 33 I believe you can get 120. (purple and exotic might be off 1 or 2 light) Now you just need to upgrade them all to get that. Everytime you upgrade the defense upgrade you get more light. I recommend playing the queens bounties right now with friends to get 2 legendary at least