As long as you don't exceed the DC rails' current potential, you can stick whatever you like in there. But second busboards are a bit much...
Instead, what I'd suggest is to use a flying bus cable that has the proper 16-pin connectors across the board, such as https://schneidersladen.de/en/eowave-flying-bus Just plug it into an open busboard connector, and there you are.
The only time you'll have too many modules is when you've exceeded the potential load of the P/S itself. If you've got a max capacity on your 12+ rail of, say, 1.2A, but your module complement only draws 750 mA, you're fine. But if you're beyond 3/4ths of the way to the rail's maximum current, then things will start to get a little tricky because you DON'T want to have a 1.2A supply dealing with an 1.2A load, and your inrush current at power-up can momentarily EXCEED the current capacity of the P/S and this can harm the supply. But since that's the only ACTIVE power component, and things like busboards and cables are passive, you only need to worry about the P/S load overall.