There are a number of different commands you can use for switching to a
buffer by its number. For example, if you want to switch to the buffer
assigned 7, you can do :7b or :e #7. However, there is a quicker way
than typing out either of those commands.
You may already be familiar with CTRL-^ for switching to the alternate
file which is generally the previous buffer. If you precede that command
with a number, you will switch to the buffer with that number instead of the
alternate file.
From normal mode
7 Ctrl-^
will switch to buffer 7.
See :h Ctrl-^ for more details.