There are no unit and bg restrictions. you may have 30 of the same champ. 30 spells. Any faction. All factions. You may only vs another person playing a custom bg. Aimed at people who like to have fun, mess around and make silly combos.
Ohh mighty @Senshu ! I beg of you to make this happen! - Give me the strengh to troll other guys, since i'm so bad at everything else in this game! Give me the pleasure.. Give me the fun... Give me 15x Skeezick Netter & 15 Skeezick blades in one BG!
We are looking into expanding the options available in Custom Games, but there may still need to be some restrictions.
10 of every faction would be a very funny outcome. (or the old 20 rune decks, for quicker casual games!)