Transformers are usually rated in VA. The power rating is the same on both the primary and secondary windings.
To calculate how much current you can draw from the transformer, just divide the VA rating by the secondary voltage:
A 10VA / 12V transformer will supply 0.833A.
For dual secondary transformers, the rating is for both secondaries combined. To get the maximum current for each secondary, divide by double the voltage:
A 60VA / dual 18V transformer will supply 60 / 36 = 1.67A per secondary.