Something

ls | grep _[0-9] > tmp$$
biggest=0
while read file
	do
	num=`echo $file | awk -F_ '{print $2}' | awk -F. '{print $1}'`
	if [ $num -gt $biggest ]
		then
		biggest=$num
	fi
done < tmp$$

echo "text_`expr $biggest + 1`.rtf"