Crazy Czech numbers. BYLI would be correct if there were 2, 3 or 4 men. Anything 5 and up (including descriptive amounts like few, many) start behaving weird. This is true across all genders. More about this weird phenomena under "numbers 2" tips and notes. https://www.duolingo.com/skill/cs/Numbers-2

Basically, old Slavs counted normally only to four. After that it is an omount of... Literally pět mužů is similar to five of men (genitive mužů, not nominative muži).