how many books is a reasonable amount in a library (on average)
That depends on the size of the library. Below is information about 2 very large libraries. Try the same search for your town or city.
https://www.google.com/search?q=how+many+books+in+library+of+congress&oq=how+many+books+in+library+of+congress&aqs=chrome..69i57.9207j0j7&sourceid=chrome&ie=UTF-8
https://www.google.com/search?sxsrf=ALeKk01_Typv_gCb4AvUDTDZfn4g8ua16Q%3A1591026557239&ei=fSPVXvr7Daaa_QaQmI-YCw&q=how+many+books+in+los+angeles+public+library&oq=how+many+books+in+los+angeles+public+library&gs_lcp=CgZwc3ktYWIQAzoECAAQRzoFCAAQzQI6BAghEApQo8UBWNrTAWCo1QFoAHACeACAAXyIAb4JkgEDMi45mAEAoAEBqgEHZ3dzLXdpeg&sclient=psy-ab&ved=0ahUKEwi6rpbm--DpAhUmTd8KHRDMA7MQ4dUDCAw&uact=5