Addressable RGB LED Art Project

What @DrAzzy said,

will the first LED of the second row be chained from the end of the first row?

Yes, that's what the code above assumes you'll be doing. If you do something else you'll need to change the code.

Just to add that it'd be a good idea to chain the signal but to connect the power and ground together all down the edges of the lights - otherwise you might find that when they're all on the power draw is so high that the ones right at the end will be more dim than those at the start.