How many clients are needed for consensus?

Hi everyone

After a block is processed, the result will be recomputed by other clients for consensus. My question is:

1. Is my understanding correct?
2. How many clients are needed for consensus? I mean when Ethereum determines ''well, enough clients validate the block, so the block can be appended in the blockchain''? I think not all clients in the network are required, otherwise consensus may run into problems if some clients crashes or run very slowly.

Thanks for your responses.