The best hashtags are usually just a few words. 1-4 is a good length to shoot for, but in general, you should try to keep your hashtags as short as possible. This makes them easier to remember, and easier to use on platforms like Twitter, where a 280-character limit makes brevity absolutely essential.
For example, don't hashtag a whole sentence. Just write your caption and choose a few keywords that stand out and use those instead.