More nodes generally have less time between earnings, when all earnings are combined. As an operator you don’t earn at a higher APY – you have more frequent earning events.
Node substitution is random. As more Nodes join the network, the interval between earning events increases.
Consider a single node. With the current number of validators (around 2700), a single node may have earning events once every two to three weeks.
Now consider operating 8 nodes. Each individual node would still have the same random interval for earning events - about once every two to three weeks. But with 8 nodes, all earning at different times, you would see a more steady stream of earning events. But you might see an earning event every few days: e.g. Node 3 earns on a Monday, Node 1 on Thursday, Nodes 6 and 2 on Saturday and Node 1 again on Sunday.
The APY will still be about the same. It doesn’t scale much. The majority of the cost is in the stake. Some savings can be created by operating multiple Nodes on a single host. However that cost savings is not as significant as it was once was, with the rising value of the PRV stake.