Explain how you can show a certain number of products for guests in Magento?

Create static block and using single category id which product display and given number as you wish here example after create static block magento :-

{{block = "catalog/product_list" category_id="3", num_product="" limit="certain number here"  template="template path here"}}

Hope this help

Create a static block with number of product you want to show. then identify that the user is login with is_login() function.so function will be like Is_login() {//Static block code

}hope that work!!!