Are there upload size or count limits I'll slam into?

The realistic ceiling is your plan's hot storage allowance rather than some arbitrary per-upload cap — the platform is designed for very large sets and advertises uploads of thousands of images at once. Real-world examples of very big albums exist (again, that ~400 GB wedding), and for extreme cases the team has been known to personally help photographers get large collections uploaded — one testimonial mentions the founder helping via WhatsApp. So the honest answer: the software scales, and when you're pushing absurd volumes, there are humans available too.