NOTE: there is no photobook workflow for mobile
Yes, you can add multiple pages when in the workflow you can add pages in multiples of 2 down in the bottom right corner of the “Design Pages” screen.
Next Day Photobooks
4x6 (20 pages only)
Double-sided Photo Books - 6x8" (15x20cm), 8x8" (20x20cm), 8x12" (20x30cm) – 20 to 40 pages
Deliver to Home or Store Photobooks
Premium Hardcover Photo Books - 6x8", 8x8", 8x12", 12x8", 12x12" – 20 to 100 pages