My (paid) AI says it is a non-existent rule. It traced the Migration Law through to article 51, which apparently states that students have the right to work and places no limit on the amount.
It goes on to a detailed analysis of the matter, looking at various DMN dispositions and finds no limit. It then looks at labor law, and finds nothing. It then discovered that someone who enters on a transitory visa to study (which is a different category altogether--as per Steve's information--for short term fellowships and exchanges and the like) does have to seek approval from DMN to do so (but again no upper limit appears), so that is probably the source of part of the confusion.
It goes on to a detailed analysis of the matter, looking at various DMN dispositions and finds no limit. It then looks at labor law, and finds nothing. It then discovered that someone who enters on a transitory visa to study (which is a different category altogether--as per Steve's information--for short term fellowships and exchanges and the like) does have to seek approval from DMN to do so (but again no upper limit appears), so that is probably the source of part of the confusion.