[Mia]

It all depends on how you want to manage your schedule. My recommendation is to assign the individual's resource according to the time they can dedicate to your project (i.e. a full time person would be 100%, someone working on two different projects equally 50%, etc).

The dates will change depending on how many resources you assign as long as the resource setting is 'Keep Effort Fixed'. This means that for a task that needs five days of effort:
-if you assign one 100% resources, the task will take five days;
-if you assign two 100% resources, the task will take two and a half days;
-if you assign ten 100% resources, the task will take half a day.

If you wish to keep the duration fixed and the amount of effort to reduce, there is an option for this too. It means that, if you have a task that takes five days of effort:
-if you assign one 100% resource, the task will take five days;
-if you assign two 100% resources, the task will take five days but both resources will be delivering ten days worth of effort.

If you select the option "Keep effort and duration fixed", the work will be shared amongst your resources. It means that, if you have a task that takes five days of effort:
-if you assign one 100% resource, the task will take five days;
-if you assign two 100% resources, the task will take five days and OmniPlan will share the work so that only 50% of each resource's time will be used for the duration of the task.

I'm not usually good at explaining things in text, did this help?
Mia