How to add the content of compactingRules in the config file。
If I want to add multiple rules, how should I use the syntax, how should I add content between "<" and ">"

The config file explains how the setting is used. However it depends on what version you're using, because it was modified between 1.16.x and 1.19.x to correct a wording issue.

This is the text in 1.19.2:

#List of rules in format "domain:item1, domain:item2, n".
#Creates a compacting drawer rule to convert 1 of item1 into n of item2.
compactingRules = ["minecraft:clay, minecraft:clay_ball, 4"]

So if you want to add another rule to this, you'd do it like this:

compactingRules = ["minecraft:clay, minecraft:clay_ball, 4","minecraft:block, minecraft:item, 4"]

As in, each "entry" in this list, is a comma separated double-quoted string.

I don't know what you mean with "<" and ">"