I assume that each device should have it's own keypair, but it is only my assumption. I recall that HDCP somehow "does not work as intended" (whatever that mean

I assume that each device should have it's own keypair, but it is only my assumption. I recall that HDCP somehow "does not work as intended" (whatever that means) when both devices have same key. And as for the ~50 number, that seems likely, I vaguely recollect that the required number was 40 (probably for 50% chance of success), but I don't remember the exact details and assumptions for this attack, so I take "at most thousands" as safe overestimation. Also I expect that time complexity of this attack (which is probably not exactly fast, as it entails solving pretty large system of equations) decreases with more known keys.