A bucket of lava contains one "square unit" of lava - if you pour it into a trench that is longer than 1 block wide, it will spread out.
There's a maximum amount of spaces it will flow over before it just "evaporates," so it might take some experimentation to determine how many buckets of lava you need in order to fill an arbitrary width trench to the depth you desire.
You can test your trench by tossing in something that isn't worth much, say a copper coin or a piece of wood or dirt. If there's lava in the square where it lands, but the item survives, you've hit the right depth.
I have more experience with lava in older versions of Terraria, but in previous versions it was possible to fill a bucket with less than a full unit of lava. You can use this to duplicate lava, for instance. Pour a full bucket out over a 2 or 3 block wide trench, and then before it completely settles, click the bucket on it again. You'll leave a half a block of lava, and gain a full bucket's worth.