How to control number of items per row using media queries in Flexbox?

I had to get rid of the margin around the blocks, because percentage widths are difficult to cleanly apply to elements with margins, but you can see the changes at http://codepen.io/anon/pen/jPeLYb?editors=110 :

@mixin max-width($width) {
    @media screen and (max-width: $width) {
        @content;
    }
}

.container {
    position: relative;
    display: flex;
    flex-flow: row wrap;
}
.item {
    background-color: tomato;
    box-sizing: border-box;
    padding: 20px;
    outline: 2px solid blue;
    flex: 1;
}

@include max-width(992px) {
    .item {
        flex-basis: 25%;
        background-color: red;
    }
}

@include max-width(640px) {
    .item {
        flex-basis: 50%;
        background-color: green;
    }
}

The important parts here are:

• flex-flow: row wrap which allows the flexbox to appear on multiple rows (the default is nowrap)

• flex-basis which is the equivalent of width in this case

• position: relative which makes the widths relative to the container, rather than the body (this would screw up the rounding)