Q: How much data can be cited in a review paper?

For an original article, the literature review should roughly be around 15-20 percent of the length of the article. However, for a review article, the approach is different. A review article does not have a separate literature review section. It contains an introduction, a main body, and a conclusion. The body of the article includes an extensive and in-depth analysis of the existing literature. You have not mentioned whether the article you are reviewing is a narrative review or a systematic review. For a narrative review, it might still be okay to summarize some of the studies, but the main purpose of the article should be to critically analyze the studies based on the research question that is being explored, not just to summarize them. 

For a systematic review, the body will consist mainly of a detailed methods section, where you mention the inclusion and exclusion criteria and then go on to explain methodological approaches, models and/or theories, the extent of support available for a particular thesis, studies that agree with it as opposed to studies that disagree, etc. Since review articles do not involve collection of data, it is natural to cite data from existing literature as supporting evidence for their rationale.

There is no industry accepted limit to determine how many works or how much data can be cited in a review article. To find out the allowable limit of citations, you will have to check the guidelines of the journal.

To understand what to expect from a review article, do go through these resources:

• A young researcher's guide to writing a literature review
• A young researcher's guide to a systematic review