Steel/Electric actually has the most resists with eleven (normal, electric, grass, ice, flying, psychic, bug, rock, dragon, steel and fairy) and immunity (poison)
For what it's worth Steel/Fairy is second, with nine resists (normal, grass, ice, flying, psychic, bug, rock, dark, and fairy) and two immunities (poison and dragon)
Extra
There is only four Steel/Electric Pokemon (magnezone line and togedemaru)
Source
https://pokemondb.net/type/dual