Tomato plants grow in levels, and each level is called a truss which consists of one or maybe 2 sets of flowers and then fruit which comes from the same level on the plant. Then you would normally get another layer of leaves and then another truss.
A truss will have a varying number of tomatoes on it depending upon the variety and you pinch out (cut off) the growing tip after 6 or 7 so that the plant can then focus on ripening the fruits rather than putting out anymore flowers.
I would suggest that 6 or 7 trusses was quite low, 8 or 9 is probably more like it, although depends on the year and so maybe for your first year stick to 6 or 7 and be happy with a good crop.
Make sure you feed them weekly with a tomato feed and also keep a close eye out for potential fungal infections which appear as purple blotches on the leaves. Spray with fungicide as soon as you see these or you will lose most of the leaves and then the fruit.
To increase the number of plants.
After pinching out any side shoots place them in multi purpose compost.
They will root giving you a lot more plants.
One year I bought 3 tom thumb plants in the spring and grew them in a basket in the garage.
By planting some of the pinched side shoots I had dozens of plants all of which fruited and some I gave to a relative to grow in a hanging basket.
They provided a constant supply of small sweet tomatoes all summer and autumn until the first frost.
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