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 and three test satellites on the network. If in a particular month your nodes got traffic from all 6 satellites, you will receive 6 payout transactions per node. So if you were running 4 nodes, you would receive a total of 24 transactions per month. The number of production and test satellites may vary over time, according to the demand on the network in different regions and testing needs.

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.