Two Border Patrol UH-60 Black Hawk helicopters patrol the southwest border region
That is about the last big part that would ever get barrier, if we built out every mile of the border.
Building barrier (and patrol road, and installing utilities) on the steep parts of mountains, seems to be something that we have not yet cracked the code on doing, in anything close to a cost effective way. The great bulk of what is being built in California, Arizona and New Mexico, is the flat areas between the mountains. There are hundreds of miles in Texas that are remote and very rugged. "Nothing but miles and miles, for miles and miles", as they say. Here is the Santa Elena Canyon in Big Bend National Park. The border is the middle of the river:
The Comprehensive Plan calls for about 1,100 miles of barrier, along a border that is 1,954 miles long. Some of that 1,100 miles may be counting secondary barrier, so there would be even some fewer number linear miles with border barrier. I estimate about 100 miles of secondary barrier - maybe more. They have announced long runs (~30 miles) of secondary barrier already in the Yuma Sector in Arizona (Barry Goldwater Test Range). So perhaps under a thousand linear miles, out of 1,954 would get barrier.
221 mile of existing barrier is assessed as effective so perhaps under 800 linear miles need new barrier. The total Trump Program that is currently funded, is for 731 miles (primary and secondary combined, which includes the 170 completed). Another $3.6 in FY2020 Military Construction (MILCON) funding has been announced as intended for diversion to the Wall, but it has not yet been formally transferred, and is the subject of lawsuits. So the Program is roughly fully funded, to put barrier in about half of the border miles - everywhere that the exhaustive analysis indicated it is needed.
Last year's MILCON was fought all to the Supreme Court. The Supreme Court allowed the spending to go forward (Presidential Emergency and all) until the lawsuits were finally settled - but they are still underway, and new suits oppose this year's MILCON spending.
I think when they get close to finishing contracting out the billions of other dollars waiting to go to work, the FY20 MILCON will get transferred, right on cue. Last year, the Supreme Court had to weigh in during July, (on an emergency expedited basis) to lift the lower court injunctions. This year could be a similar schedule, as the lawsuit has already been at the Ninth Circuit for some time already.