How to download alots of page by twenty-twenty

Question

I have an app that download a books, where each book can contain a random pages. So I coded a logic using threading:

while pagenum < (int(Book["total_pages"])+1):
    tlist = []
    for a in range (20):
        if pagenum < (int(Book["total_pages"])+1):
            x = threading.Thread(target=ThreadDownloads, args=(queue, pagenum, destination))
            tlist.append(x)
            pagenum += 1
    for x in tlist:
        x.start()
    for x in tlist:
        x.join()

The ThreadDownloads functions is download a specified pages and it's working great, so the problem is this: it ran ThreadDownloads 20 times but it downloaded only a part of the pages:

• pages between 1 and 7 but not those between 8 and 20
• pages between 21 and 29 but not those between 30 and 40

I dont think my ThreadDownloads function is working correctly because before applying threading it was running fine.

Any help is appreciated.