Is praying for twins for righteous offspring considered an transgression?

There is no harm in supplicating to Allah to bless you with righteous twins. This is not considered an transgression in supplication. Rather, keenness on having abundant offspring is encouraged in Islamic law, as the Prophet, peace and blessings be upon him, said: "Marry the affectionate, fertile woman, for I will boast of your multitude before the nations."