Ah, so you mean the command history, and not the scrollback. In the future, if you aren't sure about the terminology, you should take more care to make it clear what you are referring to.
In bash, the command history size is controlled by the environmental variables HISTSIZE and HISTFILESIZE. Other shells probably have similar settings.
There are other useful settings as well, such as HISTCONTROL, where you can set options such as ignoring duplicate entries. The man page for bash will tell you more about them.
BTW, <20 commands is incredibly small. My bash man page says the default is 500 lines. Strange.