We recommend starting with some basic math. How many pages or products do you have multiplied times the number of words on a page. Its best to add about 20% to this number to account for those extra pages like terms and conditions or about us pages. You can paste the text from a page into an online tool like wordcounter.net to determine the words on that page.
For example if you have 100 pages with 250 words each and added 3 extra languages:
Yappn does a great job at not replicating translation so your Home link that appears on every page will only count as one word regardless of the number of pages it appears on.