You can assign multiple resources to a task without a problem. Many users are not aware of the effort-driven flag that is set for each task. If the flag is turned on, the task will get shorter in duration as each resource is added. For this reason some people feel that only one resource should be assigned per task. Turning off the flag per task will resolve this issue. (Double click on the task –> advanced tab –> removed the effort-driven check mark).
I have worked with clients who have assigned 100+ resources to a task. Go ahead and assign as many as you need without concern.
I also would like to know where you saw this because it should be corrected if possible.