In theory, the lesser the number of Portfolios, the better it is.
According to the true Portfolio Theory Approach, you should have Search and Content in one Portfolio. And depending on how they contribute to the objective that has been set, the Media Optimizer will figure out the best way to spend across the two.
But search engines sometimes make algorithmic changes to one of its networks and not the other. To prevent changes on the content network from affecting spend on search, and vice versa, some advertisers prefer to create separate campaigns for search networks and content networks.