How much weight will 1/8" plywood sustain for compression?
You appear to mean compression perpendicular to the plane of the plies.

Depends a good deal on what the plywood grade and material is, and the conditions it's operating under. 

http://www.g-lumber.com/pdf/APA_Plywood_Design_Specification.pdf Suggests that between 340 and 160 lb/in**2 for dry service and 105-210 for wet service (for grades appropriate to wet service) so you'll need a fair number of square inches (which you have inconveniently left out of your question) to support 50,000 lbs.

Looks like 476 square inches on the low end to 147 on the high end (of loading) though any sensible person would derate a bit more for additional safety factor, or use 1/8" steel as a shim rather than 1/8" plywood. That's 3.3 square feet to a bit over 1 square foot.

8 feet (96inches) by 8 inches = 768 square inches, resulting in a loading for 50,000 lbs evenly distributed of 65 pounds per square inch, so per the above document you should be safe at that loading.