I am certainly not an expert on the technicalities of server/client communication via OPDS so I'll leave the technical answers to someone else.
However, I don't think it's a numbers issue in any way. I also have a very large collection of books - several thousand - on my Calibre server and have had no problems with any of those header feeds. If those other feeds, like Gutenberg, are working correctly, then I would think that it has something to do with how you have Calibre setup for your server. I would start looking there.
Are you running Calibre - and accessing the OPDS - from
within your home network? I seem to remember having problems if you're OPDS address is set with 192.168.1.xxx as your Calibre server needs to have a static IP address. There are several threads in the Calibre forum that I had to mull over to figure out how to set mine up.
If you are trying to access your home OPDS from out in the internetz (as I do from all around the world) you would need to make sure you have the correct address as well...I use a dynamic DNS name service (such as FreeDNS although there are several ones out there) that allows me to put a name as my OPDS address which is automatically updated whenever my Internet Service Provider decides to change my home's IP.
eg.
//myhomeopds.freedns.com:8080/opds
instead of
//29.42.31.67:8080/opds or
//192.168.1.25:8080/opds
I hope that at least points you in the right direction. Maybe someone else will have more details.
Cheers,