Even a single, solitary tooth gap can lead to chronic tooth loss as a condition. This is why dentists make such a big deal of trying to save every single tooth in your mouth, regardless of how bad a shape they are in, because once an extraction happens, the teeth next to the tooth gap will start to get loose, unless the tooth is replaced. This is the main reason why dentists stress so hard the importance of dental implants, because they are the only method of tooth replacement that ensures that all of the bits of the tooth, including the tooth roots, are replaced. Only like this can chronic tooth loss be prevented.
The alveolus is a ridge of soft bone tissues that lie beneath the gums, and are responsible for keeping your teeth stably in your head and anchor them to your jawbones. The alveolus is only around if there are tooth roots in it, and if the tooth roots are gone, then the alveolus is absorbed by the body. When this occurs, all of your teeth are compromised, as part of an organ that is keeping all of them in place is no gone, weakening the alveolus itself. This means that missing teeth as a phenomenon can lead to missing teeth as a condition.
Other problems that can occur after you have a missing tooth are related to the deteriorating conditions in the mouth. Because the tooth is missing, the adjacent teeth have to pick up the slack, and frequently the tooth opposite of the one that is missing will become ruined really quickly, or the same tooth on the other side may also experience adverse effects, as you now have to chew or bite solely on the other side, as the missing tooth cannot be used. This can lead to many problems, not just tooth decay: it can lead to TMJ syndrome, which is a disease of the jaw joints, the uneven force can lead to neck and back pain, to jaw pains and possibly even digestive problems from not chewing your food properly.
Although stopping tooth loss as a condition can only be had by getting dental implants, if you are not yet ready for that kind of financial commitment, there are other, lesser solutions you can apply to the problem to try and deal with it. You could get a crown or a bridge if numerous teeth are missing, and get them anchored to adjacent teeth in order to regain chewing function. Once this occurs, it is bets to try and save money for a dental implant.