Memory usage in a DAW is subject to many factors, including:
Track count
Audio vs instrument tracks
Project length
Sample rate/bit depth
Usage of sample-based instruments
Buffer size and multicore settings
Plugin count and types used
That being said, 16GB is fine for most projects. Of the above, sample-based instruments eat RAM the fastest, but even then they can be streamed from disk in realtime given how fast the new MBP SSDs are.
Most likely, if you are not using orchestral VIs to score a feature-length film then 16GB will suffice.