Is the # of tandem breakers limited by NEC?

[iwire]

Celt

or where single conductors or multiconductor cables are stacked or bundled longer than 600 mm (24 in.) without maintaining spacing and are not installed in raceways

Single conductors that are not installed in raceways.

To me single conductors that are not installed in raceways describe the single conductors that are installed in enclosures.