20mbit/s per second is not so much, but it should work. In general you have to differentiate between three things with Lemmy:

1. Users on your own instance directly utilizing the client api (medium bandwidth heavy, medium CPU)
2. Users from other instances indirectly accessing communities (low bandwidth, but potentially high CPU)
3. Either of the above directly loading images from your instance (potentially very bandwidth heavy)

For 1: you can limit the number of people you allow to sign up on your instance (highly recommended)

For 2: this is depends on the number of subscribers and the number or external servers but overall is relatively efficient as the remote servers take most of the load, but your server will still have a high database load if remote people comment and post a lot.

For 3: Until Lemmy implements a fully image proxy, this will likely be your main problem with only 20mbit/s upload. But you can severely limit the size of images being uploaded and convert them to smaller sized webp images automatically. But if a lot of people try to download simultaneously the same image from your instance, it’s going to be a problem.