On which mountain did John see the Lamb standing with the 144,000?

Answer: Mount Zion

John saw the Lamb standing on Mount Zion, and with Him the 144,000 who had His Father's name written on their foreheads.

“Then I looked and saw the Lamb standing on Mount Zion, and with Him 144,000 who had His name and His Father’s name written on their foreheads.”— Revelation 14:1 (BSB)