It's impossible to say just how many comments will increase your hub score, and same with questions and responses and sharing and so on. Your hub score should go up when you increase participation on hub pages but you have to be sustained in your efforts or nothing will happen.
So have a strategy ready. Say for the first week you target 5 questions and 5 hubs - give answers and comments - and your hub score remains the same. Repeat the strategy but increase to 6 or 7. I'd try this for a month. If you get no joy then I should look to your hubs to see if you can increase production or quality. It's an ongoing effort that should yield results.
Best of luck.
In my experience the hub score goes up very slowly but comes down very quickly!!
Don't be too depressed if your hub score doesn't move! If your hubs have good original content and you are active on hub pages your hub score must go up.