The number of words that are in a 5-minute speech depends on how fast the speaker talks, but usually averages between 600 words and 900 words. This is based on the speaker talking at a rate of two or three words a minute.
Very slow speakers who are very deliberate with each word might only achieve one word per second. This means they would only get through 300 words in a 5-minute speech, but this is considered quite a slow rate of talking.
Fast speakers may talk at a rate of four words per second, and very fast speakers can get up to five words per second. This is at the extreme end of the scale, however. However, for those who speak at a rate of four words per second, a 5-minute speech will need to have 1,200 words. Very fast speakers who get through up to five words a second will need 1,500 words in their speech.
Things that affect that affect the rate at which a person talks includes their natural vocal delivery, the audience, the occasion and the context. The advice from experts is not to go for an exact word count though. This is to be used as a guide, whereas practice will tell the speaker the actual length of the speech.