Limit number of portfolio items in isotope (all category)

Hello,
I’m developing a website using the Reel theme (homepage isotope).

This question has probably been already asked but I can’t find it anywhere.

I need to limit the number of portfolio posts to 12 in the ALL category.
Could a kind soul inform me what would be the most non-intrusive way to do this?

Thank you in advance for your support.
Viktor