Validators are agents that will run your dApp. They are interested in your application to get validation rewards out of transaction volume your dApp creating, you are interested to pick right set of validators (right shard) to get best compromise between cost of execution and security of network. The more validators are in network and the more powerful they are - the more costly will be their services, because the network provides high throughput and high level of decentralisation. The cheapest shard is a single RNode hosted by yourself, but that's a no good solution for working with important data.
See here the current status of the validators: Validators