Sspx, I would say could be considered a denomination. It’s certainly not a Sect.
There are three Christian Sects. Catholic, Orthodox, and Protestant.
Since by definition Catholics are in union with the Bishop of Rome, SSPX (although closely affiliated and working toward full communion) for purposes of census would have to be considered a Protestant denomination.
Further Lutheran synods are most definitely different denominations. By the way you are classifying denominations you may as well say everyone is catholic and there is one denomination since we have the same core belief that Jesus is God and died for our sins.
That doesn’t address the problem of do many churches.
Going back to Lutherans, you have done supporting homosexual marriage and some not, things like that make for fundamentally different theologies.
Of course one must also ask how to classify Evangelicals and non denominationalism.
Some non denominationalism churches have thousands of members and for all practical purposes are their own denomination.
Other Evangelical Free Churches are similar.
One can do a google search for church in a major metropolitan area and get thousands of results of vastly different churches with vastly different beliefs.