I think you mean "32 or 64"
A warp consists of either 16 or 32 threads grouped together.
I thought a Wavefront on AMDs architecture consists of 16 execution units. Or am I wrong? (I just used warp as a general term, because I like it more )
Nah there's 64 threads in a wavefront. In their latest architecture (GCN) the SIMDs are 16-wide, but they execute each instruction 4 times to complete it for the entire wavefront (so a single-cycle instruction actually takes 4 cycles to execute).