Validator sets can only get so large before the messaging complexity gets in the way, causing a direct reduction in throughput (TPS). For the unsharded Babylon Mainnet version of the Radix Public Network, a validator set of 100 provides a good compromise between redundancy and throughput.
In the Radix Xi’an release, Radix will move to a fully sharded ledger and an effectively unlimited number of validators.
Further reading: