how much is too much potassium sorbate?

So I picked 8 lbs of raspberries and made a 2 gallon batch of wine. Its been fermenting for almost a week now. Last night the SG was 1.009, and it tasted perfect so I wanted to stop fermentation. I added 2.5 tsp potassium sorbate (bottle calls for 1 tsp per gallon to prevent renewed fermentation when sweetening).

It was still going strong this morning so I added another tsp. Tonight the airlock is still bubbling away.... do I add even more to stop it, or will more start to introduce off flavors?

Adding potassium sorbate WILL NOT stop an active fermentation, it does not kill the yeast, it will however keep the yeast from reproducing. I would not add anymore to the wine. Wait for the fermentation to stop and see where your gravity is, you will most likely have to back sweeten to get back to 1.009.

To TRY to stop an active ferment you typically cold crash, rack, stabilize, sterile filter and typically keep wine in a cool climate. Difficult but some people accomplish it. Or you cold crash, rack, and fortify with a high proof liquor. Or pasteurize.

But in all reality the best thing you can do is allow the wine to ferment to as dry as it is going to go and then stabilize with k-meta plus sorbate and then back-sweeten.

That much sorbate can add an off-flavor to your wine, you definitely won't need to dose it with any additional sorbate at this point, unless of course your sorbate was old and expired.