why Threads_connected falls down automatically in some cases

How can I monitoring, when Threads_connected increase from which place(files) or when it decrease, because some of cases it dose not fall down automatically in our production server and we are getting to max connection error there, So i need to identify issues in which files has problem, My site in magento 1.9 and max_connections=151 so i want to know what the ideal count for that and how can i monitoring ?