Hi @Helge ,
based on this, shared disks aren't supported for Standard HDD at all:
Currently, only Ultra Disks, Premium SSD v2, Premium SSD, and Standard SSDs can be used as a shared disk
Reference: Share an Azure managed disk - General limitations
Why it says Max Shares
= 5 in the screenshot you posted .... I have no clue ;-)
(If the reply was helpful please don't forget to upvote and/or accept as answer, thank you)
Regards
Andreas Baumgarten