If you would like your dahlia plant to grow more stems, then top the dahlias!
When the plant is 30-50 cm high, cut out (or pinch) the middle shoot to 1 cm above the 3rd leaf pair. The plant will then branch more widely into a sturdy, bushy plant that produces more flowers. This so-called 'topping' always seems a bit scary to do, but rest assured the plant will soon sprout again and you will be rewarded with even more flowers.
Another TIP: Cut dead dahlias regularly. Cut the stem with the dead flower to a new leaf or to the first bud. In this way, the plant's energy will go to making new buds and allowing new flowers to bloom. If you leave the old flowers on, they will produce seed pods and that is a waste of the plant's energy.