Is the Roll-Off of Butterworth Filter of degree N always N*20db/dec?
Yes. Rolloff slope is part of the definition of a Butterworth filter.

If you have passband/stopband specifications and filter order specification, then it's likely that your problem is over-constrained. You have to loosen at least one of those constraints of passband/stopband/order so that the filter can be designed.

Or, you can use a different filter type.