The original post asked three questions. The answers given do cover the first question, "When I open a terminal window, which shell is opened by default?" They also answer a question which was NOT asked, namely "How can I find out which shell is currently running in the terminal?" However, as far as I can see nobody has answered either the second or third questions originally asked, namely "How do I check how many shells are installed?" and "How do I change the shell used from my account?"
- To answer "How do I check how many shells are installed?" the following command will list all the available shells:
For example, on a default installation of Ubuntu 18.10 this gives:
# /etc/shells: valid login shells
However, by default sh is a symbolic link to dash, while rbash links to bash with the option -r ("restricted bash") so there are actually only two shells, not four as the above list suggests. The following command will show you whether any of the listed shells are in fact symbolic links, and if so where they link to:
ls -l /bin