Yes, you can group together similar items and value them as a group if they were donated to the same charity on the same date.
For example, say you donated 15 hardback books. You can just list that as one item with the description "15 hardback books" and the value as 15 X $2 = $30 total value.
Things that aren't similar should be listed separately. Don't group together a toaster with a kitchen table set.