Actually, we can customize the length of columns in the grid system using the source code of Bootstrap and Sass. Read here
First, create a scss file named myBootstrap.scss and type in:
$grid-columns: 10;
Next, download the source code of Bootstrap and import it in the scss file:
@import "bootstrap-4.3.1/scss/bootstrap" // bootstrap.scss is the file you need to import.
Then, we have this in myBootstrap.scss:
$grid-columns: 10;
@import "bootstrap-4.3.1/scss/bootstrap";
Finally, install Sass and compile it into myBootstrap.css in the command line or you can do it in your way:
sass myBootstrap.scss myBootstrap.css
Now, just replace your original css of Bootstrap with myBootstrap.css:
<link rel="stylesheet" href="myBootstrap.css">
Done. You can read more in Theming Bootstrap