"You planted your tomatoes too close together." Have you heard that from visitors to your garden? Not leaving enough space between tomato plants is one of the most common gardening mistakes. The good news is that it's probably not too late to move them if you do it right away.
It's so easy to look at those small tomato plants and forget how big they will get during the summer. Even after all these years of gardening I still have to resist the urge to plant them too close together.
Large commercial farmers who grow tomatoes for canning plant their tomatoes very close together. We often see them in double rows about 18 inches apart with the plants very close together in the row. The double rows are then spaced around four feet apart. Planting them like that allows farmers to get the maximum production from their land. Special varieties have been developed to allow them to be planted so densely. Those varieties also account for the difference in taste between tomato products. It's why one brand of ketchup tastes different from another.
These young plants look far apart but will eventually grow to fill in the space.
Home garden varieties won't grow and produce well if grown too close together. Older heirloom varieties will take up much more space than more modern varieties. They'll keep growing all season long and can get pretty big by the time the first tomato is ready to be picked. That type of growth is referred to as being indeterminate. I plant those varieties about four apart in cages and up to six feet apart if I let them sprawl over the ground without any support.
Plant breeders have developed tomato plants that will stop growing once they reach a certain size for their variety. Those are referred to as determinate, we used to call them "bush tomatoes". Determinate types of tomatoes can be planted closer together, around three feet apart in cages.
Good air circulation reduces disease problems and adequate sunlight allows the fruit to grow and develop. I remember hearing a saying many years ago that went something like: "Plenty of air and light, grows tomatoes right". Giving your tomatoes enough room to grow will provided them with the air and sunlight they need to thrive.