the best situation is to have on each node only one shard per index
Well. Yes as you can be sure that all the power available on this machine will only be used by your single shard.
But that's a waste of resources in most cases. A single node can fortunately hold much more than one single shard.
Depending on what you are doing you can start with a number of shards equal to the number of cores you have (rule of thumb), then increase that number and see if you still meet your requirements.