How much islamic knowledge and hadeeth has been lost throught the ages

I was told recently by a mufti that imam ahmad ibn hanbal memorised 1000000 saheeh hadeeths. Now i dont know of any sources where there are 1000000 saheeh hadeeths. Bukhari apparently has approximatly 2000 saheeh hadeeths. So how much knowledge still exist percentage wise if possible or any answer sheding light on the matter would be accepted.