I was wondering this while DMing my little bro with the red box set as he was fighting the white dragonling. With my sentinel druid, would I be able to tame multiple creatures and possibly even use multiple at once?
As a Sentinel Druid you get 1 animal companion that is based on your chosen season. This is the only animal companion you can get. There are however ways to get other animals to follow you around (though they won't count as your animal companion so you can't use them for any of the attacks or other class features you have that require your animal companion).
The simplest way is to use the Train Animal aspect of the Nature skill. This allows you to train normal animals to do things like "Come", "Fetch", and "Stay".
The next simplest way is to take either the Fey Beast Tamer [DDI] or Animal Master [DDI ]themes. These allow you to gain a Fey Beast or Animal Minion respectively, and a power related to the chosen companion.
The hardest way would be to take a multiclass feat for any Arcane class and Arcane Familiar [DDI]. This will allow you to gain a familiar many of which are animals, these familiars give you different benefits based on which one you pick.
Note in addition to not being able to use any of these additional animals for you class features that require an animal companion, they also share your actions. This means that if you have both you animal companion, a fey beast, and a familiar you still only have 1 each of a standard, move, and minor action to split between all of them and yourself. For this reason it is generally a good idea to limit the number of animals you have following you around, since generally you will be doing things with your animal companion and the rest of your pets will be left not doing anything but taking up space.
You can also do a hybrid summoning druid beast master ranger. This gives you an animal companion, the ability to change shape like them, and ability to summon others to join you.