"You can currently create up to 20 Lightsail instances, 5 static IPs, and 3 DNS domain zones in a Lightsail account."

So obviously there's a hard cap within one account. The $80 account comes with 5tb, so one would have to be burning a bit of bandwidth - 100tb - to cap it out. Frankly though, at $1600, Amazon is still printing a massive profit margin on that 100tb of transfer.

These seem like they'd be good front-end servers hooked up to RDS etc.