How many monthly payout transactions should I receive when I run multiple nodes on the same payout address?

If you are running multiple nodes, you should be using the same common payout address for all of these nodes, as per section 3.d.ii. of our Storage Sharing Terms and Conditions. The monthly payouts you receive in your common payout address are totals per satellite for all the nodes you are running on this address. We do not sent payouts separately per node. Currently, Storj Labs operates four Satellites on the network. If in a particular month your nodes got traffic from all 4 satellites, you will receive 4 payout transactions, each containing the total earned for all nodes that had traffic coordinated by that satellite for that month.

So in summary, payouts are per satellite, as each satellite does its own calculations of payouts only for nodes connected to it, and groups them by payout address.