Let us assume each lega of the Letter 'H' takes 8 LEds and the dash between the lines take 6 LEDs. Make a series parallel combination such that each series combination takes 12V with or without series resistors. Connect the anodes to + 12V and the cathodes to ULN. If you have 3 series branch of LEDs for the Letter H then each branch current will be 20 ma. 60 mA will be the max current. Each channel of ULN can sink 500 mA. If all the 5 letters of HELLO take approx 60 mA * 5 = 300 mA then all cathodes can be connected to one o/p of ULN. better use NPN TIPxx transistor which can handle 1A.