If you've purchased the PDF + Ebook Pro plan on Pressbooks.com, you received 250MB of storage when you upgraded your books. If you purchased the Ebook Pro plan, you received 25MB.
This is the total storage limit for all the text, media, and other content that makes up your book. The size of your book grows with each image or chunk of text that you add to it. You can continue to add content to your book up until you meet the 250MB quota.
Most users never reach 250MB, so in most cases you should have more than enough storage. In general, text tends to take up relatively little space.
However, images and other uploaded media can take up more space. We recommend keeping media uploads between 1-2MB. This means your book can likely have around 120 - 250 images before meet the storage quota, depending on the file size of the images you're uploading and how much other content is in your book.
Please note that Pressbooks does not offer any storage expansion packages.