This feature is only available in the desktop editor.
This article will show you how to change the number that a numeric list starts on.
Select Start Number List At...
After you create a new list, add your text (in the example below, I added "Step 3"), and then select Start Numeric List At... by going to Format > List > Start Numeric List At...
This is a terribly weak solution. There should be an option to continue numbering from previous list. Suppose I have five groups of numbered steps interrupted by 5 screenshots. Not only do I have to manually supply a number for four of those, but if I add a new step early on, I now have 4 sets of steps that must be manually corrected with a significant risk that one gets missed and numbers are off. Please add a "Continu numbers from last list" option.
@Eric: I'll be sure to pass your request on to our product team for consideration!
I second Eric's request for a "continue from previous list" option.