#container {
    padding: 10px;
    display: flex;
    flex-wrap: wrap;
}

#container div {
    width: 20px;
    height: 20px;
    border: 1px solid black;
}

/* #container>div:nth-child(17n+1) {
    width: 100%;
    border: 0;
    height: 0;
} */