It actually depends on your server subscription plan whether can process this notification unlimited request to your server.

If I recall correctly, I remember some of the Webhosting providers will restrict the number of requests per day, so if your site has more than 1k users which means that it probably will send more than 1k requests to your server every minute then you have to calculate the estimate how many requests need to send to your server per day.

And this "Use Single Polling Request" feature is not supported for all the servers, it might have some issues but you can give it a try to enable this and then the only drawback is the notification polling will not work if the user opens more than 6 tabs on the browser.

If your server doesn't restrict any number of requests per day, I think you do not need to enable this "Use Single Polling Request" setting, so you can set your polling interval to 5 seconds.

so if your site has more than 1k users

hello. You mean 1 k users online or total?

1k users online.