Common as hell.
Even altitudes are for westbound aircraft (180-359 degree course). Odd altitudes are for Eastbound aircraft (000-179 degree course). So it is not uncommon to have opposite direction traffic pass directly under or over you by 1,000 feet. Kinda exciting when a 747 buzzes by that close. All the planes are flying the same altimeter setting above 18,000 feet so that everyone is flying the same pressure level, and no altimetry errors occur. There’s also some other rules involved in this airspace called RVSM (reduced vertical separation minimum).
Above 41,000 aircraft must be separated by 2,000 feet vertically. So Eastbound altitudes are 41,000, 45,000 and so forth while west is 43,000, and 47,000. Everyone is flying a pressure level above 18,000 feet so we refer to them as flight levels and 36,000 feet would be called FL360.
Above FL410 traffic is fairly sparse and you can talk ATC into letting you fly an altitude that is wrong for direction of flight such as maybe heading East at FL430.
When you’re really high up in the mid 40’s traffic often appears to be above you before it goes under you due to the curvature of the earth.
Hope that helps.