What are the best practises to decide whether a variable is categorical?
The number of categories within a variable does not matter whether a variable is categorical or not.

Categorical variables are mutually exclusive, unordered groups. For example, Christmas and Halloween are different holidays but have no order in the concept of "holiday-ness".

Ordinal variables which are mutually exclusive, ordered groups with no consistent measure of the distance between the ordering. For example, ranking items (e.g., first, second, third, …). There could be big (or small) differences between each of the places.

Numeric variables have consistent differences between values. The difference between 10 and 11 miles is the same as the difference 20 and 21 miles because "mile-ness" is a consistent measure.