1/2 campden tablet is plenty to treat the full amount of around 34 ltr for a target 23 ltr brew length.
Unless you have
chloramine in your water you don't need to use campden tablets. Very few water companies use chloramine; I obtained a list a few months back but obviously that will change over time. For the time and bother of contacting your water company and getting an answer you may as well drop in 1/2 a tablet.
Assuming no chloramine, to remove
chlorine let the water stand for 24 hours/overnight. However, heating water to mash and sparge temperatures is enough to drive off chlorine e.g.
What are the various ways to remove chlorine/chloramine from tap water?:
"
I heated the water (about 1 gallon) and measured its chlorine for each 10C (20F), and when reached about 60C (140F) the chlorine disappeared, before boiling. Just an empirical result."
I brew therefore I ... I .... forget