You can use the "random" expression to create a random number:
for example random(10) will generate a floating point random number between 0 to 9 (0.5, 2.0, 7.87, etc.)
if you want only integers use floor( random(N) ) this will generate random integers between 0 to (N-1)
if you need to convert those to text (string) use the str() expression:
str( floor( random(10) ) )