How many grams of sugar does it take for insulin spikes?

A non-diabetic can eat as much sugar as they like without seeing spikes. This is because their pancreas will just pump out the necessary amount of insulin.

I have Type 1 diabetes, my pancreas does not produce any insulin. 10g of carb will raise my blood sugar by 3mm/ol or 54 mg/dl. Which means that anything more than 10g will cause a spike in blood sugar. I don't have any insulin spikes because my body doesn't produce insulin.

And I have to correct two things in what you wrote.

First, it is not sugar that causes insulin spikes but all carbs.

Secondly, most cereals have about 30g of carb per serving.

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The glycemic index number is more important to know what will cause a sugar spike. High GI foods will cause a spike, low GI food will not. Do a Google search for glycemic index for more information.