I would agree with Karen that the plant generally requires a larger pot (though you can continue trimming it back). I had 3 of them growing on the patio for several years in 14 inch pots. They do tend to grow sideways and that is actually the reason they are not there anymore.
Because the spines on that plant are among the most hazardous in the cactus group. Technically they are not spines but glochids. Whatever, the little hairs all over the plant (which give it that great look) get stuck in your skin really easily and require quite a bit of attention to remove.
So be careful where you put your plant, especially if it starts growing sideways, because there is a bit of a potential hazard to consider.
