On day 1, your SMS limit will be capped at only being allowed to send 200 messages. Once you hit 200 messages sent out in a 24-hour period, your limit will increase to the next level. The SMS limit will refresh every 24 hours. If the account is brand new then each day the increments will increase according to the table above. Once you have hit the 8th day, your SMS limit will be capped at 5000 per day.

I'd like to send more than 5000 SMS per day, how can I increase my limit?
Once your location(s) hit the 8th-day mark (5000 SMS per day), you may reach out to support and request a Limit extension.
No, you cannot respond manually to incoming messages. SMS daily limits will affect all messaging activities including manual SMS in conversation, automation within workflows, and bulk actions.