To add a slider question, go to flow creation page and select Slider:
After editing the text, go to settings: 
From Advanced Settings, you can make the question required, set a lower and upper limit (For example, enter a value between 50-75), or set the step interval.
The step interval shows the changing range each time you play the cursor. For example when you choose 20 it will go 20 by 20 like so:
0-20, 20-40, 40-60...
If you choose the step 5, it will go 5 by 5
If you leave it blank, it works as a default 1 by 1
It looks like this on the flow screen:
