graphics card - Getting more than two NVENC video streams

I'm attempting to record (to a file) three or more 1080p webcams on a single computer simultaneously.

I successfully used two instances of OBS studio to record two webcams. The third instance throws this error:

"Starting the output failed. Please check the log for details. Note: If you are using the NVENC or AMD encoders, make sure your video drivers are up to date."

Digging deeper I found on the Nvidia NVENC wiki:

Consumer targeted GeForce graphics cards support no more than 2 simultaneously encoding video streams, regardless on the count of the cards installed. Professional cards support between 2 and 21 simultaneous streams per card, depending on card model and compression quality.

Both of these sites were helpful, but didn't provide the information I needed:
NVIDIA VIDEO CODEC SDK
Video Encode and Decode GPU Support Matrix

What video card or what configuration of video cards do I need to be able to get more NVENC video streams?

This Nvidia forum post indicated that video cards not in sli would each contribute to the encoding. However, the wiki link quoted above indicated that this would not give more NVENC streams.

Answer

The professional series cards that they are referring to are the "Quadro" branded cards.

These cards tend to be significantly more expensive than their GeForce cousins, but are aimed at CAD, scientific and heavy duty transcoding operations. They are loosely based on the same hardware, but have less limitations and drawbacks.

With a basic one of these cards you should be able to transcode two or more streams, but newer or higher end cards may be able to transcode more streams at once.

The reason you only get a "between 2 and 21 streams" limitation is because it depends a lot on the compression level and the video size of your incoming and outgoing data, whether you are using hardware decoding as well as encoding and other factors.

You might be able to encode only two 4k h.265 streams at once or twenty one 720p h.264 streams. It depends is about the best you can get.

You might be able to rent some time on one of the cloud GPU providers to find out how many you can get out of a professional card at your settings, buying a top end Quadro only to find that the basic one could have done the job is an expensive mistake and it is doubtful any suppliers would have sympathy.