There is no definite answer to this question, and is purely depends on your situation. Search engines, and in particularly Google, are getting much smarter and their technology can detect the difference between plural and singular phrases. The correct answer to your query is probably to carry out some primary keyword research to determine what your customers are actually typing into search engines. Use a keyword tool or employ a SEO agency to undertake some findings and analysis on your behalf.
Once the keyword research stage has been completed, the next stage is to think about your content. The best approach to writing content is to make it compelling and engaging for your target audience. If it’s possible to insert your keywords into the text naturally, this is a great approach. However, do not try to add keywords into your website copy for SEO purposes, as there are more important signals in a search engines algorithm than keyword density (and this has been the case for a number of years). In the perfect scenario you would include your primary keyword, plural/ singular variations, synonyms, and other related language into the text; but this should come naturally as the content should be contextually relevant to the primary keyword you are looking to target.