There are very few limits:
Your downloaded files are stored in your account for 30 days. However, there is no storage limit. You can save as many files as you want. For downloading you have 10 parallel download slots available, which means you can download 10 files in parallel.
However, despite the unlimited storage space, we would appreciate it if you deleted files that you no longer need. This saves us an enormous amount of space and allows us to offer this unlimited space.