How many writes do I have on my 250GB SSD hard drive on my Macbook Pro High Sierra
You cannot in general track how many writes exactly have been performed on the drive. You would have had to keep track of that statistics yourself somehow.

Similarly you cannot in general say how many writes are left, as there's no "guarantee" that a drive will fail at a certain point. Nor is there a guarantee that a drive won't fail up until a certain point.

The best way to get an approximation of what you want is to look at the SMART attributes on the drive. You can do that with the program "smartctl" from "smartmontools". If you have homebrew installed, you can install smartmontools with this command:

brew install smartmontools


Then afterwards run it like this:

smartctl -a disk0


(replace disk0 with the actual physical disk you want to examine - you can find the device names in Disk Utility)

In the output from smartctl, you'll want to look at the so called "Wear Leveling Count". The raw value at the right tells you how many erase/program cycles each block on the SSD in average has undergone. The erase/program cycles are what wears out the drive eventually.

In addition, the number in the column "value" will tell you in basically how large a part that number of erase/program cycles is compared to the expected number of cycles that you drive can sustain. Basically this number starts at 100% for a new drive, and counts down to 0% where you would expect the drive to start failing. As mentioned before, drives can and will fail before that point - and they might not fail at that point. However, it is one of the better indicators you have of drive health.