I'm 5 days post partum, is 25ml for first pump expression considered low supply??

Congratulations on your newborn! Milk usually kicks in 4th day post partum, so to be able to pump 25ml on your 5th day is great! What you will need to do is continue pumping every 3 hours to stimulate more milk (if you're fully expressing). Otherwise, if you're latching to keep latching on demand. Do remember if you're latching to monitor your LO's wet diaper.