Yes, and the maximum that you can reach is the maximum of the combination.
Few examples:
- The number of SS cannot be more than the number of Rx or Tx antennas (both on AP and client)
- If you have a 3x3:3 AP and a 1x1:1 client, you will only get 1 spatial stream in the combination. A two-stream client will get 2 streams, and to get the full potential, you will need a 3x3:3 client as well. A (fictional, I don't think they exist) 4 stream client would only get 3 streams.
- Most smartphones are 1-stream devices, some are 2-stream.
- 3 stream devices are typically larger devices like laptops.
- For 802.11ac wave 2 with MU-MIMO: If you have a 4x4:4:3 AP and a 1x1:1:1 and a 2x2:2:2 client, you will get 1 stream from the AP to the 1x1 client and 2 from the AP to the 2x2 client. Note that MU-MIMO only works from the AP to the client, not from client to the AP in 802.11ac.
For further reading, there should be in depth (going pretty deep technical) in the referred Outdoor MIMO VRD and the 802.11ac Whitepaper that is referred in that document.