A sentence should be long enough to reach its end, however, you still have to maintain the aesthetic of your writing, as well as balance. Your sentence can be one single word, or it can be extremely long, but what will best engage the reader and retain attention?
In most cases, it is better to write a sequence of sentences of a natural length rather than designate a specific amount of words you have to use in a sentence. A succession of sentences that are of equal or similar size will tend to distract the readers. The same applies to a series of sentences where the word count is too divergent. You have to make sure that you don’t constantly burden your sentences with different forms of parenthesis. This means injecting phrases or words that involve the use of dashes, parentheses, and commas.
In general, when writing sentences, try not to overthink, as it’s best to write in a natural flow. However, when you start proofreading or revising your work, pay close attention to the length of your sentences, and if you feel like some are too long or too short, feel free to separate or join them. This would, of course, depends on how you want to showcase specific sentences in your writing.
If you are looking for a rough estimate of how many words a sentence should have, a ballpark figure of 20-25 is a safe bet. Although, the literacy of your reader will often dictate the average length.