In most cases, libraries buy individual "copies" of digital titles from publishers. Publishers often require that each digital copy is only loaned to one person at a time (just like physical library books). This ensures that authors, narrators, and other creators are paid fairly for their work, even if it's consumed digitally.
The number of copies the library buys depends on a few factors, like demand and budget. Even if there's a lot of demand, the library's budget might not allow for more digital copies of a popular title. Most libraries have limited budgets that cover their physical collection, programming, and more, in addition to digital titles.
If all copies of a digital title are checked out, you can place a hold and wait for it to become available. If the wait list is long, let your library know. They may be able to purchase more copies, if their budget allows.