i have cubase pro 9.5 do i need wavelab for mastering ?
Depends, but in general: No.
You have Cubase. What do you think you need additional? Sometimes if you have to ask, it means you don’t need it.
If you mean commercial or professional mastering, you will need more than what any DAW offers including Cubase. How you get the additional tools and functions is up to you. If you mean “finalizing” then Cubase or most any other DAW IMO can do the job.
Cubase is a DAW, Wavlab is a mastering environment. Yes there is overlap, but there are a lot of unique functions too.
Things I use Wavelab for that are not available in most DAW’s including Cubase:
Wavelab-Cubase exchange (not quite as nice as it was about 15 years ago but it works), built in redbook, CD assembly via montage (including gap lengths or even no length with PQ codes etec) detailed audio analysis, spectral editing, error correction, smart bypass, batch processing, batch re-naming, basic audio CD or detailed audio CD, Data CD & DVD, CD text, DVD audio, very good metering (the best 3D frequency analysis) , audio file comparing, meta normalizing, and DDP.
Weak areas for Wavlab IMO would be automation, (PG says this will change soon) and don’t buy Wavelab for the factory processors/effects etc. They are good, but that is not what you really want Wavelab for. A lot of ME’s use Wavelab only for capturing and assembly.
It’s best you read threads about the differences and decide for yourself whether or not you need Wavelab.
I do all of my mastering in Cubase. There are plenty of plug-ins to cover the bases.