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In general, you probably shouldn’t have as many references as in a full manuscript. A poster is not intended to be a complete literature review. The point of references is to give an indication that you’ve done some Academic research while preparing your poster. Audiences will rarely want to copy down the references to check up on them, which is often a major point of references in a paper.
Experiment with different ways of incorporating references. Always check with your lecturer or supervisor.
Some links that may be of use. Do note that some of the following guidelines are for APA which is a version of Harvard but the main points are still of use.