I am a little confused, I am using Messages but after like 20 messages the previous messages disappear. How can I increase it or go back and look at previous messages?
Messages are saved in the User ~Library/Messages folder on your computer, messages from any device. Attachments too. This makes for a very nice backup and reference that can, in addition, be easily read with QuickLook.
They're loaded automatically when scroll to the top of the conversation. If nothing is loading it's because there is probably no history for this conversation.