This is not Only a Software Limit, its a "limit" related to the Resources and the Know of the user of the limitations.
Related to the "think" that nvenc have 0 affect of the card. That only is true if nvenc encode the framebuffer.
The patch for remove the limit to open it to the Pro card level limit, is only work problem free in special conditions on consumer cards related to the physical limits. in a default use way of OBS with a Quality Output 2 encode and above works "only" without additional loads or very low workload.
So to tell 3 or more is default working is totaly wrong, its related to the workload.
And related to the posted log, there is limited to 2 not by software driver or chip, Simple by workload
16:01:21.772: [jim-nvenc] init_session: nv.nvEncOpenEncodeSessionEx(¶ms, &enc->session) failed: 10 (NV_ENC_ERR_OUT_OF_MEMORY)