step down transformer 20-0-20
When a center tapped transformer is labelled with a particular current rating for its secondary, the number actually stands for the capability of the current carrying capacity of the copper wire in the secondary. 

This means the each side of the transformer output is capable of delivering the labelled current.

In case of 20-0-20, 6A, if two outer ends of the transformer winding is connected to a bridge rectifier and the center tap is not used and no filter caps are installed,  you will have 40 volt @ 6A (RMS).

If you use only one outer end and the center tap and connect it as above, you will have 20 volt @ 6 A (RMS).