Sure you can completely secure the border with less guards with the wall than without (you'll still need a lot more guards than we have now to secure the entire border), but the wall will cost so much money that could just skip the whole thing, take the money you save and hire enough border patrol agents to make the existing fences effective.
Then you don't have to deal with the awful PR image of a modern developed nation building a wall along the border.
The government spent more than $7 billion building a fence that only covers a fraction of the border. How much do you think it's going to cost to build an enormous wall that covers the whole thing?
Oh but wait, it won't cost anything because Mexico is going to pay for it. The wall isn't a real proposal, it's just Trump pandering to Republican voters. Just like he's now supposedly pro-life after years of being ardently pro-choice. Or how he is firmly for gun rights after years of supporting the assault weapons ban. Or how he loves reading from Two Corinthians.
It starts at the beach and goes into the mountains. When it was built peiple that crossed the border moved put into the desert and mountains.