The TC wants to ensure that the amount of assignments by validators to parachains is at a maximum in order to reduce the possibility of a network issue: This proposal ensures that before the next runtime upgrade.
Right now the way we have validators assigning on Kusama is overzealous: everyone is assigning to check the 1 parachain Kusama has. With 2 chains, we expect 1/2 of them will assign, and so on. We're already at the maximum of assignments, so the TC wants to correctly track the 40 parachain groups we have on the Network (200 validators / 5 per group) by the next runtime upgrade to ensure finalization.
Once we deploy the new changes + a node update, we can start to bring this value down (to something like 5 or 10).
This proposal:
Preimage can be found HERE.