You want to make sure the realtor you are dealing with is knowledgeable in the area you are looking to purchase. You should have multiple realtors if you are looking in multiple cities but only use one if you are looking in one area. For example you would only need one realtor if you are looking in Vancouver but that realtor probably wouldn't be much use to you in Surrey.
Do not use more than one Realtor in one city. If you are looking at purchasing a residential property, all realtors have access to the exact same information and you will only get yourself into trouble, not to mention a lot of confusion, by using more than one realtor. They work really hard for their money and feel taken for granted by clients who are playing the field. If you are loyal to them, they will be loyal to you. If that realtor isn't working out, fire them and find someone new that you like and trust.