First rule - forget about measurements.
If you like lots of Yorkshires, one egg per person does two large-ish Yorkshires. If it's only a little extra for form's sake, one egg per two people will do.
Crack the eggs into a bowl. Add plain flour a bit at a time and beat in well (with a whisk) until it gets to be about the consistency of ready-mixed Polyfilla - a thick yellow dough. (Do *NOT* use self-raising flour, otherwise you get Yorkshire cakes instead of Yorkshire puddings! My housemates at uni did this to me once by "borrowing" my plain and self-raising flour for some cooking, and then shovelling the self-raising flour spilt on the worktop back into the wrong bag. :-)
Then add a *tiny* bit of milk (about a dessert-spoonful) and mix that in with the whisk until it's smooth. Then another dessert-spoonful and mix again. Then maybe two dessert-spoonfuls and mix again. Then a bit more, and so on. The point is that the egg-and-flour dough won't absorb liquid easily, so if you add all the milk at once then you just get lumps of dough sat in milk. The only way to get this mixture smooth is to mix a little bit of milk in at a time, until the dough finally becomes a thick liquid. Then you can top up with milk as needed, until you've got your finished batter mix.
How much milk? Well, you want the batter mix about the same consistency as single cream. Don't worry too much about this though - it doesn't really matter. (Note: Pancakes follow the same recipe, except there you keep adding milk until the batter mix is about the same thickness as the milk you're adding. You don't want it that thin for Yorkshires though.)
Then beat the batter mix well. At this stage you can also add anything to season it. A bit of salt is pretty much a must. I like to add a bit of rosemary, thyme and black pepper as well - that gives the Yorkshires a taste of their own to complement the gravy, instead of just being a bland floury taste. You need to beat the batter mix so there's a nice lot of bubbles on the surface - this adds air throughout the mixture, which is what makes them rise.
Heat the oil in the trays in the oven. When it's hot, take the trays out and use kitchen roll to wipe it all the way round the trays. Then half-fill the holes in the trays with batter and put them back in the oven to cook until they're brown. Oven temperature isn't critical either. Job done.