4x
of these make a nasty Incendiary Lance (or Omega units, as I remember them being called in the fiction).
For 'mechs, I commonly army my SHD-2H's with Infernos, due to the Shadow Hawk's ultra-cool heat scale. Of course, a mere SRM-2 doesn't do much, but every little bit can help.
For purpose-built Inferno lances, however, I tend to use a modified Crusader packing 6 SRM-4 launchers, two MLs, and 18 heat sinks. But my fooling with MegaMek suggests that Infernos are a much better deal for things like the Warrior VTOLs and all manner of fast, light vehicles (such as Scarborough's Saracen & Scimitar). 'Mechs can make great use of Infernos, which is why the SRM-4 has become one of my favorite weapons, but they're much more a force-multiplier for vehicles, and the utility of inferno-armed Jump Infantry is very difficult to overstate.
