A few thousand, would be a best guess.
Some basics:
When is my work protected?
Your work is under copyright protection the moment it is created and fixed in a tangible form that it is perceptible either directly or with the aid of a machine or device.
Do I have to register with your office to be protected?
No. In general, registration is voluntary. Copyright exists from the moment the work is created. You will have to register, however, if you wish to bring a lawsuit for infringement of a U.S. work. See Circular 1, Copyright Basics, section “Copyright Registration.”
What is the registration fee?
If you file online using eCO eService, the fee is $35 per application. If you file using Form CO, the fee is $50 per application. Generally, each work requires a separate application. See Circular 4, Copyright Fees.