I realize that this is an old post. However, if you see this reply, please take a look at the following link:https://blueirissoftware.com/
The software supports up to 64 cameras and is reasonably priced when compared to the cost of a conventional proprietary NVR. I believe that it supports ONVIF . If I recall correctly, there's a 21 day trial period. It would be best to purchase the software from the developer in support of his maintenance efforts, even though you may find it a bit cheaper elsewhere,
Be sure to take a look at the PC requirements before you decide to go with the software. That's why the trial period is nice. Basically, you need a dedicated higher end PC with an Intel processor, such as an I7 or I5, about 8GB of RAM, and a Western Digital purple drive that fits your budget. Many people have opted for an SSD drive for their OS and a WD drive for video storage. Again, check these specs out for yourself before committing to anything.
Truth be told, I have not yet purchased a good machine on which to run this program, so I'm using it on a lesser grade one while also running other programs. Plus, I only have a few cameras. The more cameras that you have may require more RAM. To sum it up, my setup is less than ideal, but, it does what I need it to do at present. Hopefully, with the correct hardware you'll have a good system. Good luck with your project.