How many easter eggs are there? so far, i have found :

EP 1: Companion cube in the background

EP 2: Turret jacuzzi

EP 4: Chell apparition in vent

EP 5: Bloody text with skulls on the top of the map, and turret birthday

EP 6: Undertale cheese reference

EP 7: Rubik cube solved, star wars collage, chell painting behind target

This is without counting chell sightings(the one in the vent is special, so i count it) and thinking with portal.com murals.