If you work with external translators, they probably ask you for details about the amount of work. They need this information in order to give you a quote.
On the projects list there already are counts of segments/keys left per target language. But, in reality, most professional translators base their pricing on the number of words to translate.
You can now get this information directly on Translation.io using the contextual menu of each project:
A window looking like this will show up:
Here you have the total number of words at the top and, for each target language, a computation of the number of words left to be translated.