Less than you'd think, because you likely wouldn't need full "Texas coverage", you mostly need the birds that actually show up. BirdNET filters by location, you'll likely need a couple of dozen. Over the summer I've had about 40 visitors where I live.

Right now the coverage of North America is relatively sparse, but the interest has been high and I'm hoping for some contributions.

Edit: For North America the most obvious source is Audubon's Birds of America, which is public domain and should provide a lot of coverage. This collection includes some beautiful plates.