Hello!
If you mean - units in rack (free space) for better cooling, so:
X440 have side to side passive cooling, so you can don't leave 1RU free space between 2 switches in rack (also X440-8t more narrow then other switches).
A lot X440 swithces normally work with minimal free space between switches.
BUT also there is document - "SummitFamily HW Install Guide" -> Chapter "A Safety Information", which says "Establish at least 3 inches clearance on all sides for effective ventilation" - so it's about 1,5RU.
Thank you!