You certainly don’t see every single bird that visits the area of your yard, so how do you get a rough headcount? Experts disagree on details, but they’re in the same ballpark. Count the birds you see in your yard in a typical ten second span (count those at your feeders, flying nearby and those just watching). Do it several times and take the average. Multiply that average by 5 or 6 (that’s where they disagree) for a very approximate number of birds who look to your yard for support. his method figures in the birds you don't see - those that are deep in the vegetation, in a nest or high in a tree.
Be as ignorant of property lines as the birds themselves. They could care less about who owns a certain tree or hedge. “If you can see it, you can count it” should be your guideline.