<ul>
    <li>//</li>
    <li>//</li>
    <li>//</li>
    <li>//</li>
    <li>//</li>
    <li>//</li>
    <li>//</li>
    <li>//</li>
    <li>//</li>
    <li>//</li>
    <li>//</li>
    <li>//</li>
</ul>

sCss
ul{
width: 1200px;
display:flex;
 box-sizing: border-box;

li{
 width: calc(100% / 6);
 border: 1px solid red;
}
}

the border that appears in the above code causes some of the borders in the middle to be 2px;

so add

 margin: 0 0 -1px -1px;

but the addition of this codeword results in 6px on the right side of each line;

is there any good way?

one of my own solutions is not very friendly

&:nth-child(6n) {
   width: calc(100% /6 + 6px);
}