Would anyone know a Wi-Fi analyzer that would show me how many 802.11ax MU-MIMO streams are happening in real-time or happened during a capture between an AP and a few clients? Trying to test how many simultaneous MU-MIMO streams an AP will handle.
I would guess that there isn't anything on the market that will do exactly what you want. That is not to say that some R&D group at a chip maker doesn't have the gear to do it. Saavius sold their OmniPeek line, so you'll have to find the current company and ask them. Talk to the actual customer support people to get the best information on their products.
Part of the problem is that being able to log such data, requires more horsepower than it does to process it. simple logging can bog down many devices that handle the data just fine.
With a multiple of the correct USB /n/ac adapters, products like OmniPeak are able to decode single device multi-stream transmissions.
You may be able to ask the chip manufacturer for the data to prove their claims, but unless you are a really big company, willing to spend big bucks and sign NDA's you probably won't be able to get real data.
You'll probably have to rely on your own throughput test data, and make estimates from what you find.
You can, of course, use a utility like iperf and measure the throughput. Using the rates in the 802.11 standard, you'll be able to calculate the number of streams.
Having a really good NTP server will go a long way to helping you compare logs from different devices.