The key to training tomatoes to a vertical support is illustrated in the picture below.
Tomato plants branch by growth of buds that form at the Y between two existing branches. The bud in the upper panel has been removed in the lower panel by simply pinching it off. These buds grow amazingly fast in warm weather, so buds need to be removed weekly. Left unchecked, each bud forms a branch that will form more buds and branches, so the number of vines grows exponentially, from 2 to 4 to 8 to 16, etc. If you remove buds properly you will end up with a single vigorous vine, which will easily grow 6′-8′ or more up a support.