I use a collection of 2x10 lengths with beveled edges, mixed and matched and stacked as needed.
You won't ever need six ramps--at the very most five if the rear were low and side-to-side leveling is also needed. There's never any need to raise the highest wheel up even further to get level.
For side to side leveling, it's also often possible to level with only the back or only the front wheels if it's not too far out of kilter. The wheels do not need to all be coplanar. Thus, four ramps would be sufficient for most situations. (If not, parking facing the "wrong" way if possible will help reduce the number of ramps needed, though often at the expense of convenience.)