Scrum guide lists few rules and recommendations to be followed as part of the framework.
Some of the very important rules in scrum are:
The length of the sprint should be 30 days or less. That means, you could have a 1 day sprint or a 30 day sprint and call it scrum. That also means, that you might have a 35 day sprint and are seeing all the benefits of empirical process control, but you cannot call it scrum.
Another rule and the most important rule in Scrum is, Every sprint should deliver a done increment. If you don’t have a done increment at the end of sprint, you are not doing scrum. That is the core artifact expected in scrum that allows to inspect and adapt and if you don’t have it, your entire feedback could be flawed that will make things worse.
Another rule, Every event is timeboxed. That means that’s the maximum amount of time you should spend in any event is fixed. If you can achieve the outcome of the event in half the time or event 1/4th of the timebox, that is great. As a Professional scrum master you need to ensure this boundary of a timebox is respected by the scrum team. This timebox allows the scrum team to live the scrum values, to focus on the expected outcome.
Now on to an example of a recommendation mentioned in the scrum guide
The typical size of a scrum team is 10 or less members. This was updated with new version of the scrum guide as now it talks about the scrum team, that is, it includes the Product Owner and the scrum master in the count. This comes as a recommendation in the scrum guide, that means you could still be doing scrum if you have more that 10 people in your team, but it comes with a caution that in larger teams, the complexity, the dependency, the churn could be considerable for an empirical process to manage effectively.