The brain does not generate distinct "thoughts," so it is impossible to calculate how many thoughts the brain has per second. In addition, there are a number of subconscious processes that occur in the brain at all times.
The stream of thinking people seem to hear in their brains is only part of what the brain is doing at any one time, and inhibitory processes prevent people from being conscious of all of these thoughts. The human brain is always active; when people sleep, their brains continue generating the same types of thoughts as when they were awake. Activity in certain regions, however, slows considerably.