the limit is the A16's capabilities only - 16 positions if I understood the sparse information available
The A16 can render virtual speakers at at most 16 different positions simultaniously, but you can measure more positions to create several different speaker layouts. For example for Auro 3D the height speakers should ideally be in different locations than for Dolby Atmos. In particular the Auro 3D center front height (30 degrees above the center speaker) and the so called "voice of god" (straight above the listener) are totally different from what occurs in the Dolby Atmos height speakers.
Even though the A16 initially can not decode Auro 3D, you can measure an Auro 3D speaker layout and connect for example the pre-outs of an Auro 3D capable receiver or pre-pro to the analog inputs af the A16 and enjoy Auro 3D that way.
Also the optimal surround speaker placement for 7.1 (90 degrees pure side surround) and 5.1 (110-120 degrees) are different. Ideally I would want to have both available.
Soon we will know if the A16 offers the promised possibility to combine speakers from different PRIRs/BRIRs into one preset. [Edit: we now know this is indeed the case.]
If that is the case We don't have to measure for example the front speakers again for each format to create a complete PRIR for each format. Instead you would have to measure each position only once. For example make 1 PRIR with all 11 "
ground ear level" speakers you would ever need. (center 0°, front L,R 30°, front wide L, R 60°, side surround L R 90°, alternative surround L R for 5.1 120°, surround back L R 150°) Another PRIR with all 6 Atmos height channels and all 6 or 8 (I don't know if Auro 3D will ever need surround back height speakers) Auro 3D height channels. With that the user can then create a preset for each format, choosing speakers from the different PRIRs.
By the way: I found the Dolby Atmos speaker layouts description on Dolby's website a bit vague. They mention some leeway in possible positions for the "
ground ear level" speakers but they don't clearly state what (if at all) the consequences of that for the positioning of the height speakers are. (Dolby Atmos height speakers should be at 45° elevation).
Auro 3D is much simpler and straight forward: all the first elevation layer speakers are 30° above one of the
ground ear level speakers, and the third layer speaker is straight above the listener.
[Edit: I changed all occurences of "ground" to "ear level" because in the A16 manual "ground level" has another specific meaning: for speakers that are firing upward from the floor (used in ambisonics I guess)]