An AC server driver will give you a continuous current and a peak current.
In the datasheet of the motor there will be a torque constant figure and this is typically Nm/A so from this you can work out the peak torque that you can get from the motor when used with that AC servo driver.
However, normally the smallest AC servo driver would be 750W and if we take this as an example the peak current would be 15A and the nominal would be 4A so is 3.75 times the nominal current.
A 750W AC servo driver would be rated to be able to handle the peak current of a 750W motor for the amount of time specified within the specifications.
This is assuming the motor is rated at the 300V DC bus.
A dc servo has a lower DC bus so the current is a lot higher, so that is why you need a larger power supply.
It makes no sense to spend the money on a servo system and then cripple it with a low powered power supply.
Also as a casing point AC servo systems out sell DC by 50:1.