This site tracks it the best they can
https://www.trumpwall.construction/ 115 122 miles built (updated from Trump speech earlier), 509+ in planning stages with the aim of completing 450+ by the end of 2020.
Congress didn't want to fund it during the government shutdown and money that was eventually given was for updated facilities, so he took ~3billion from Pentagon defense budget. It was held up for a while due to a court challenge but eventually dismissed and it's likely Trump will go back to the defense budget for more funding if he's re-elected.
There was a thread in A&H the other day crying about its impact on ancient Indian burial grounds, that should speak volumes.