Quick insert of characters into a column?

[Chad]

Hi,

I'm using emacs to edit a latex file to create a table with many rows,
say 20.  I need to insert & to separate the columns, so I need a
column of 20 ampersands.

What is the quickest way to do this?  I'd like to highlight a region
(column) and then tell emacs to insert the ampersand into that region,
but I can't figure out how to do this.

Thanks for your help!

Chad