Every format I shoot (digital, 6x7, 4x5 and 8x10) has what would be considered a normal lens and a wide lens.
The wide is usually 1/2 the FL of the normal, except for 4x5 where I have a 150mm and 90mm.
After that, I'll decide if getting a "specialty" lens is warranted, such as a 75mm shift lens for Mamiya 67.
But this formula has been quite successful, and I've almost never needed any lens outside of those (self-imposed) parameters.