There is no prescribed number of words on a poster. A poster is primarily a visual medium of mass communication or marketing. This means the amount of text you are putting on a poster design should only be as much, as is absolutely required.
However, there is one caveat here. There are certain kinds of posters like educational posters, informational posters, or scientific posters which are meant to have a little more content than the average poster.
In an average poster design, for commercial or awareness purposes, care must be taken to avoid excessive text. Typically, the distribution for a poster is 20% text, 40% visuals, and 40% space.
However, when it comes to scientific posters that are meant to include more text. The typical scientific poster consists of a title and the abstract which are the most important parts of the poster visually. While title is crisp and tight, it must clearly convey the content of the paper. The abstract must contain 250 words at the max while stating the research question, methods, observations, analysis, and findings.
An academic poster must further contain an introduction, statement of purpose or hypothesis, description of the research methods, the findings or the results, and a conclusion of the entire academic paper.
Now, of course this is not your typical poster which has little to no text. The academic poster contains a whole lot more information than the commercial or artistic posters. However, the ground rule remains the same, here as well. Include only as much information as is absolutely required. Also, an academic poster is called so, when all that text is interspersed with meaningful images, graphs, or any other visual element.
Designing an academic poster can turn out to be quite a challenge. However, if you were to use the poster maker from Appy Pie Design, your job would be much easier, and the best part is that you need no design experience or inclination to start using the platform.