The short version
Rules are read once, at speed, by someone who wants to get to the chat. Anything past about six lines is skipped entirely. Write for that person, not for the edge case you are worried about.
The average rules channel has between fifteen and thirty numbered rules, several of which say the same thing in different words. Nobody reads to the end. The people who were going to break those rules never opened the channel, and the people who were not needed about four lines.
Write fewer, broader rules
Long lists come from adding a rule every time something goes wrong. After a year you have a legal document. The alternative is a handful of rules broad enough to cover the cases you have not thought of yet.
"Be decent to each other", "keep topic channels on topic", "no advertising without asking" and "follow Discord's terms" covers the overwhelming majority of what actually happens, and a moderator can point at one of them for nearly any incident.
Say what happens, not just what is banned
A rule with no stated consequence gets argued with every time it is enforced. A rule that says what follows — a warning, then a mute, then removal — ends the argument before it starts, because the outcome was published in advance.
This protects your moderators as much as your members. Consistent, predictable enforcement is far easier to defend than a judgement call made in the moment, and everyone can see it is not personal.
Put the rule where the decision is made
A rules channel is read once and forgotten. The rule that does work is the one visible at the point somebody is about to break it. A pinned line at the top of the channel where self-promotion keeps happening prevents more of it than the same rule buried at number fourteen in a list.
Use Discord's own tools
Discord has a built-in rules screening step that makes people tick a box before they can post. It is more effective than a channel because it interrupts, and it gives you a clean answer when somebody claims they never saw the rules.
Onboarding, in the same settings area, lets you ask a new member what they are here for and route them to relevant channels. A server that asks one question on arrival gets noticeably more first messages than one that drops people into a wall of channels.
Do not let rules substitute for moderators
Written rules deter almost nobody acting in bad faith. What deters them is a visible moderator answering quickly. If your rules keep growing, the underlying problem is usually response time rather than wording.
The test
Read your rules channel top to bottom at the speed a new member would. If you lose interest before the end, so did they — and everything after that point is not a rule, it is a note to yourself.
Review them when something breaks
Rules should shrink over time, not grow. When an incident happens, the first question is whether an existing rule already covered it. Usually it did, and the real failure was enforcement. Only add a rule when nothing you already have would have applied.
Every service is delivered without admin access — a public invite or link is all we ever ask for.