Call the bash builtin variable $RANDOM to produce a random integer between 0 and 32767.
bash$ echo $RANDOM
194
bash$ echo $RANDOM
7601
bash$ echo $RANDOM
17576
For more control, use let to manipulate the number range. Examples below.
For a single digit Integer:
let R=$RANDOM%10; echo $R
For a number between 0 and 99
let R=$RANDOM%100; echo $R
For a number between 1 and 100
let R=$RANDOM%100+1; echo $R