My question is how much 6x6 lumbers am I going to need for 2 retaining walls each 4 feet high and 50 feet wide?
Using 10' 6x6's, alternating ends in the middle of the next row, You would have 9 rows high(49.5") and use 5 per row. So 45 units plus any deadmen you use to tie it into the soil behind the wall.
You will also need gravel for drainage, landscape cloth to keep dirt away from the gravel and drainage pipe to run the length of the wall. Don't forget the openings in the walls bottom to drain excess water.
You tube will have many videos on the subject.
The 'angle of repose' of soil is about 35 degrees from the horizontal so if your wall is sloped this much it probably won't tip over.
If the wall is closer to 90 degrees it can tip or slide out, depending on the forces that it is subjected to.
Research this carefully - it's a pay me now or pay me later situation.