We, that limit is a real limit.
The alternatives are to combine situations where you use multiple "Derived" fields in a rule where you could have 1 derived field. For example combine two formula fields into one formula field and then change the rule to just use that 1 field, thus saving 1 of your precious 20 rules.
Also, note that if you are using a lookup field in a rule (e.g. [Customer name] , and you could instead use the field [Related Customer], that is not a derived field as it is not a lookup nor a formula field.
Other than that, post a request in User Voice and then solicit for votes on the forum to allow more that 20 derived fields being used in forms. That limit has been there a very long time, and browsers and computers are stronger now.
