it all depends on how you wire it.
if you wire them in series, then you will have 5000mAH at 12 V.
ir you wire them in parallel, then you will have 50000mAH, but the voltage will remain at 1.2 V.
edit to add:
if you really wanted 50000mAH at 12 V, you would need 100 of the batteries. (first wire sets of 10 in series, and then wire each of those sets together in parallel)