15 Cards in hand is very unlikely unless I've missed something that gets you a huge hand size boost and even then you're unlikely to both fill the hand and want to hold them all.
10 is doable with the new size boosters (Looking at Harvey Walters) but a card in hand is one you aren't using
Quite a few Seekers routinely hold 5 cards for their Educational edification however. It's not fast or free but if you're playing with a big hand anyway you might want this over the Magnifying glass (it's a Tome)
EDIT: I missed Dream Enhancing Serum which could get you some very big hands -you're probably still better off just throwing the cards at checks most of the time.