I am currently playing a full summoner Alchemist with six imps, both golems, six of my own zombies, six of my dog's (when he feels like casting them...), and five skeletons (oh, and charmed monsters when I can).
As the previous poster mentioned, constant re-summoning is the best way to 'keep track of them'. This will, of course, depend on what you are doing as a regular attack. If you are constantly attacking from behind your minions using spells or skills then you probably will not have enough mana to liberally re-summon minions. However, if you are instead either holding back, using those spells sparingly, or using a ranged weapon then you should be ok.
Remember that it is perfectly ok to try and summon more than you need. Any extra units you summon will replace older ones if they've reached their max. At the very least you will be 'healing' your minions.
The Alchemist certainly lives up to his potential as the true successor to Diablo 2's Necromancer.