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 sent out as one transaction per satellite and per node. Currently, Storj Labs operates three production Satellites on the network. If in a particular month your nodes got traffic from all 3 satellites, you will receive 3 payout transactions per node. So if you were running 4 nodes, you would receive a total of 12 transactions per month. There may be additional transactions to account for traffic from our testing satellite "stefanbenten" - this satellite is no longer being used now for uploading test data to the network, so there may not be further payouts for that satellite in future months, once all data has been deleted.

So in summary, payouts are per satellite, as each satellite does its own calculations of payouts only for nodes connected to it, and individual transactions are sent for each node.