How many pairs of running shoes and how often you buy new running shoes are heavily dependent on how much you are running - both how far and how many times throughout the week. Most runners and running shoe professionals estimate that it is best to change shoes every 300 to 500 miles. This is a general rule of thumb though, because it also depends on what kind of terrain you are running on, and in what weather conditions. If you are strictly a road runner, the amount of wear placed on your shoes will be much different than if you stick to soft trails, and still even more different than if you challenge yourself to rockier terrain.
If you are running every single day, it might be a good idea to have two different kinds of shoes. This is especially true if you change up your running so that some days you are running outside on the roads in your neighborhood and some days you are hitting up your local trails. If that is the case, then it does not hurt to purchase some trail specific running shoes, because they are designed and built for changing terrains and elevations. However, the general rule of thumb still remains: change your shoes after 300 to 500 miles. If you are running a lot, that obviously means you will be pouring more money into a new pair of shoes, more often. But if you choose to buy two pairs of shoes and switch out, you might not have to buy new shoes for a longer period of time, but you will obviously have to spend more up front to get two pairs.
Regardless of your plan, you really should not run on shoes that are too worn. And you will know when that is the case! Shoes that are overly worn do not have nearly the support and cushioning that you need, and you will notice your feet getting achier more quickly during runs, and thereafter. New shoes also help give you better running form, which means a stronger running posture and a more efficient stride and overall pace. But PLEASE do not buy a new pair of shoes just before a race. Make sure you have at least 50 miles in your new shoes before a race, so that you know they are comfortable and fit well.