Generated by SassMeister.com.

SassMeister-input-HTML.html

<ul>
  <li>one</li>
  <li>two</li>
  <li>three</li>
  <li>four</li>
  <li>five</li>
  <li>six</li>
</ul>

SassMeister-input.scss

// ----
// libsass (v3.2.5)
// ----

@mixin quantity-query( $number, $comparison: 'equal' ) {
  @if index( ('greater' 'more' '>'), $comparison ) {
    $nth: 'n + #{$number}';
  } @else if index( ('less' 'fewer' '<'), $comparison ) {
    $nth: '-n + #{$number}';
  } @else if index( ('equal' 'same' '='), $comparison ) {
    $nth: $number;
  } @else {
    @warn "Sorry, that’s an invalid $comparison value."
  }
  &:nth-last-child(#{$nth}):first-child {
    &,
    & ~ * {
      @content;
    }
  }
}
ul {
  list-style: none;
  margin: 0;
  padding: 0;
  li {
    background: #F0F;
    display: inline-block;
    height: 200px;
    width: 200px;
  }
}
li {
  @include quantity-query(7, equal) {
    background: blue;
    &:first-child {
      background: green;
    }
  }
  @include quantity-query(6, equal) {
    background: blue;
    &:first-child {
      background: red;
    }
  }
}

SassMeister-output.css

ul {
  list-style: none;
  margin: 0;
  padding: 0;
}

ul li {
  background: #F0F;
  display: inline-block;
  height: 200px;
  width: 200px;
}

li:nth-last-child(7):first-child,
li:nth-last-child(7):first-child ~ * {
  background: blue;
}

li:nth-last-child(7):first-child:first-child,
li:nth-last-child(7):first-child ~ *:first-child {
  background: green;
}

li:nth-last-child(6):first-child,
li:nth-last-child(6):first-child ~ * {
  background: blue;
}

li:nth-last-child(6):first-child:first-child,
li:nth-last-child(6):first-child ~ *:first-child {
  background: red;
}

SassMeister-rendered.html

<ul>
  <li>one</li>
  <li>two</li>
  <li>three</li>
  <li>four</li>
  <li>five</li>
  <li>six</li>
</ul>