Every league in the world has it's own history, traditions and particularities. One of the things that make Belgium unique is their club registration process. Since 1926, when the Belgian Federation was founded, clubs who affiliated themselves would receive a registration number which follows a sequential chronological order to this day.
These registration numbers became a symbol of pride for these clubs, showcasing history and longevity. Royal Antwerp for example was the first club to register with the federation and was assigned the registration number "1". This is so important in Belgian soccer that the club included the number within their crest, and they are not the only ones to do so. Rhodienne-de Hoek has the number "6" in its crest, Gent number "7", and Royal Dohain number "9" just to name a few.
It's worth making it clear that this is not a rule, very traditional clubs such as Standard Liege (#16) or Anderlecht (#35), and many others, do not have their registration number in their crest.
Interestingly, if a team folds and ceases to exist their original registration number will still be forever theirs, but there are exceptions that allow registration numbers to be shifted. For example, if two or more clubs merge into one they can choose which registration number to keep for the new club. However, the federation only allows a club to take on a new registration number after a merger if it stays within 30 km of its original location, this is to preserve the countries soccer history and protect local the fans. Furthermore, if a merger is undone the clubs involved are allowed to register back into their original numbers.
Also, as of 2017, the federation decided to allow a phoenix clubs to reclaim the registration number of its original club as long as the federation itself recognizes the new club as a true successor of the extinct club. An example of this is Lyra who was originally registered number 52, the club dissolved and was refounded under registration number 7,776 but later was recognized as a phoenix club of the original Lyra and granted number 52 again.
At the end of the day these registration numbers have practically zero impact on the field, but they are a source of club pride and a characteristic of Belgian soccer that makes it unlike any other.