I have a recipe that calls for 12 plum tomatoes (seeded, skinned, and pureed). Can I substitute regular tomatoes, and if so, in what ratio should I substitute them?
Here is a picture of what I mean by "regular" tomatoes:
The tomatoes pictured are known as slicing or globe tomatoes. The conversion rate is going to depend significantly on the size of your slicing tomatoes.
While plum tomatoes tend to be rather uniform in size, a slicing tomato can range anywhere from 1.5 to 3x the size of a plum. If they're close to the size of a baseball, that would be about the 1.5x end of the range. If they're closer to the size of a softball that is the 3x end of the range.
I'm not sure what your recipe is, but you should be aware of two properties of plum tomatoes. First, they have a much lower water content than slicing tomatoes. This makes them great for stews and sauces. Second, their seeds are rather bitter; this is likely why your recipe calls for them to be seeded.
If you're using slicing tomatoes you can omit seeding them if you desire. You'll also want to cook them longer to address the higher water content.
What would honestly be a better option is to simply use canned San Marzano tomatoes. These are widely considered to be the best tomatoes for sauces. Plus they are canned at the peak of freshness, allowing you to have ripe tomatoes year round. Fresh tomatoes bought outside of summer are often lacking in flavor.