You will need to to a query on the posts to get all of the posts as mentioned by @speakincode. Then you will need to loop over these posts and add up the totals. There isn’t a way in WP to get and calculate this without doing a query and a loop.
The only way you’ll be able to do the “last X days” part is if you also have a date field that is edited to indicate when the vacancies where added. This will complicated the admin and the query because you’re going to need some way to separate the vacancies added yesterday from the vacancies added 2 weeks ago.
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