There isn't any limit except for the fact that your hero only have 7 slots on the UI, so if you add 10 spells you will only be able to use 7 of them. (Unless they're passive.)
ehm i did some testing were i added a item spell to a unit with much abilities and one without any. the one with many had 5 standard abilities and 2 added on with triggers the one with low number of abilities had 5 standard. When i added the item abilty to both only the one with few abilities got its effect. So 7 spells are maximum thanks for your posts
i was wrong did some more testing and it was another thing that made the adding malfunction so i would now like to say that numbe rof abilties if they are item abilities are inifinit or close to
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.