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 5.1.1. of our Storage Node Operator Terms and Conditions. The monthly payouts you receive in your common payout address are sent out as one transaction per payout address (wallet). 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 1 payout transaction per payout address (wallet). So if you were running 4 nodes, you would still receive only one single transaction 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.

Please note that any pending graceful exit payouts will also be sent out in the same transaction along with the monthly payout corresponding to the month you finished the graceful exit in. So if you finished graceful exit on a satellite on July 5, you should receive the held amount along with July payouts, which are sent out in the beginning of August.