How much acceleration can a human withstand?
The specific details of duration and orientation affect the limits.
Please click the link NASA Technical Memorandum. This report describes accelerations with different subject orientations. Page 25 has a summarizing table where the range of maximum magnitudes is 2.5 G to 45 G in various conditions.
Descriptions of the effects start at page 3 and are quite interesting to read.
I hope this helps,