With this build you will have 6x undead from the necro skill lines (4 bb, 1 archer and the mender) , 4x crows from unfathomable darkness, 1 spider from shadow silk and the 2 daedra; maw and defiler making a total of 13 potential summons active at once. Recommended that you use stamina and the Stam morphs for the necro abilities to make best use of the unfathomable darkness being medium armor and use a bow for more crit to get the defiler up more often and I personally use volley and poison injection for even more passive damage on top of all the summons so I don't really have to do much. You could switch out the sets other than unfathomable though for example icy conjuror instead of defiler for magic damage and engine guardian or tremorscale for a more tank summoner. Other than the specified abilities you can use whatever else you want and as long as you have those abilities, unfathomable darkness and two other sets that give one summon each you will still be able to keep the 13 summons at once which as far as I can tell is the limit.