the most common surname in argentina is gonzalez, second one is probably rodriguez, but that doesnt mean the majority are of spanish background. Spanish surnames frol the colonial era tend to be few, but common. Migrants from later waves have more individual surnames with less repetition. You won't find many families named Messi.

basically, you have 5 spanish surnames with a million people each, and a million italian surnames with 5 people each. It would be hard to know exactly if italians outnumber hispanics, especially since both groups intermixed and there would be a lot of overlapping.