It's very simple: have a variable and increment it at each step.
Example:
cur_number=1
prefix="summer_trip_"
suffix=""
for file in *.jpg ; do
echo mv "$file" "${prefix}${cur_number}${suffix}.jpg" ;
let cur_number=$cur_number+1 # or : cur_number=$(( $cur_number + 1 ))
done
and once you think it's ready, take out the echo
to let the mv
occur.
If you prefer them to be ordered by file date (usefull, for example, when mixing photos from several cameras, of if on yours the "numbers" rolled over):
change
for file in *.jpg ; do
into
for file in $( ls -1t *.jpg ) ; do
Note that that second example will only work if your original filenames don't have space (and other weird characters) in them, which is fine with almost all cameras I know about.
Finally, instead of ${cur_number}
you could use $(printf '%04s' "${cur_number}")
so that the new number has leading zeros, making sorting much easier.