When told in essay writing to write an essay with no less than 450 words, how do I know when it is 450 words or less? This is for taking the WAEC, so it's for their marking scheme.

Standardized tests can be so hard! Especially when they are taken on paper rather than computer, estimating your word count can be difficult. The WAEC does require your essay to be a minimum of 450 words, and any words above that number do not influence your score in any way. So how do you figure out how many words you have written without wasting valuable time counting each word?
Generally speaking, one single spaced, hand-written page comes to about 100 words. Since everyone’s handwriting is different, though, you could count the number of words on three or four of your pages during your test prep in order to give yourself an idea of how many words are on your hand-written pages. If you don’t have time to do that (if you are taking the test very soon), you can count the number of words on the first five lines of your essay and divide that number by five (to get the average number of words per line). Then take the average number of words per line and multiply it by the number of lines in your essay.
This second option may still take too long if you are doing it during the examination. I recommend trying to figure out about how many words you write on a page before you go in to take the test, and then using that number to estimate the word count of your essay. If you do write about 100 words per page, then you know you need four and a half or five pages to make the 450-word minimum.
I’ve included a link to the WAEC Online webpage below. Good luck!