Shingles are cracked or broken: Shingles that are cracked can be replaced individually. However, if the crack or tear is minor, you may not even have to; you can simply repair the tile with the use of roofing sealant. Asphalt shingles can easily be damaged by high winds and falling tree branches. When replacing a damaged asphalt shingle, you can replace the entire part or cut the shingle into sections and replace only one flange.
In the end, if your roof only needs to replace a few shingles or do some minor patches, you can hire a roofer to do those small repairs for you. See 4 risks of trying to find the cheapest price for your roof replacement to prevent it from being exploited and can protect your roof investment. If your roof is more than 20 years old and most shingles are damaged or badly worn, it's time to replace it. Even if you have a storm damage claim where the insurance company is only willing to pay half of the replacement, a reputable roof contractor will fight for you and supplement your claim to get a full roof replacement.
To replace damaged roof tiles, you need to remove the damaged tile, install the new one, and seal the edges. Because both sides of your roof have a different lifespan, you'll have to pay for repairs more often than for a complete roof replacement. But the reality is that there are good reasons why a reputable roofing contractor doesn't recommend replacing half of your roof. I understand that everyone has a limited budget, and trying to save money by replacing half of the roof may seem like a good idea.