Number of post per day

Yes, as long as your site has enough visitors to support WP-Cron that often.

How can it be done? Can’t see the post limit per day option in plugin settings.

Set your days from 0900-2100.
That’s 14 hours. You want 50 posts per 14 hours that’s about one post every 1008 seconds. Set the Auto Post Schedule to every 1008 seconds.
You could also set 2 posts every 2016 seconds, or 10 posts every 10080 seconds, etc. They would all get you about 50 posts in those 14 hours depending on your visitors. WP-Cron only fires when a visitor loads your site.