Mechanical difficulty is how difficult it is to play it
Number difficulty is how easily you die or how hard it is to kill enemies
Dark Souls is mechanically easy (just learn patterns and dodge) but it has high number difficulty because you can die in 1 hit and it takes more than 1 hit to kill enemies
Mechanically difficult would be those fighting games where you have a list of 100 different button combos
Edit: Note instead of kill/die I should say win/loss to apply to more genres