Also if you summon a second creature it will cancel out the first summon, unless you have the Twin Souls perk, but by then you should already have a thrall spell, which removes the time limit.
You can also gives Staffs of Zombies to your thralls and companion, and they will then raise their own zombies.
Master Neloth in Solstheim has a staff-making machine. Does anybody know, can it be used to make a Staff of Dead Thralls? If it can, you could raise a permanent army that way.
However, when I had Serana and two Master Vampire thralls, all raising their own zombies, I noticed a definite slowdown. Not sure if this is an issue with the game itself, or just with my battered and temperamental XBox.
Probably the Xbox, mine is new and it will slowdown or lag if it has to process to much at once.