"Too Many Connections" Error

I’ve given clients a link to demo the site and almost everytime, they receive the following error message:

"Too many connections! The tunnel session “xxxxxxxxxxx” vas violated the rate-limit policy of 20 connections per minute by initiating 21 connections in the last 60 seconds. Please decrease your inbound connection volume or upgrade to a paid plan for additional capacity.

The error encountered was: ERR_NGROK_702."

Can anyone tell me what this means? How can I correct this?