Reviewer asks to add more references to already many references
You could do the following -

Compare other papers in the journal and see how many references they
have put.
Extend references by multiple references for
one instance. Example: In deep learning there is a technique called
object detection. It has 3 top notch review papers for that field
(which I know of). Instead of citing one, I would cite all three in
this case. You could do the same in your case.
You could do what Prof Santa has mentioned in the comments.
Search for relevant references from your cited papers.
Cite the general stuff. Many a times, techniques used so widely, people don't cite them. You could cite them as well.

I think 70 is actually a lesser number cites for a review paper. They generally span between the range of 150-200 in the field I work in. Though I think it differs in every field.