7 equal columns in bootstrap

Upgrading to Bootstrap 4 is probably a good option.

Comes with a class col for equal width columns

<div class="row">
  <div class="col"></div>
  <div class="col"></div>
  <div class="col"></div>
  <div class="col"></div>
  <div class="col"></div>
  <div class="col"></div>
  <div class="col"></div>
</div>

https://v4-alpha.getbootstrap.com/layout/grid/#equal-width

Tips: If you want to break columns in new lines when screen is lower than md, add this div after each column:

<div class='w-100 d-md-none'></div>

.w-100 breaks to a new line and .d-md-done hides the div on screens wider than md

So it would give:

<div class="row">
  <div class="col"></div>
  <div class='w-100 d-md-none'></div>
  <div class="col"></div>
  <div class='w-100 d-md-none'></div>
  <div class="col"></div>
  <div class='w-100 d-md-none'></div>
  <div class="col"></div>
  <div class='w-100 d-md-none'></div>
  <div class="col"></div>
  <div class='w-100 d-md-none'></div>
  <div class="col"></div>
  <div class='w-100 d-md-none'></div>
  <div class="col"></div>
</div>

An almost equal, quick solution. Without custom css.

.cell {
  border: 1px solid black;
  text-align: center;
  flex-basis: 33.3333%;
  flex-grow: 0;
  flex-shrink: 0;
  -webkit-box-flex: 0;
  -webkit-tap-highlight-color: #0000;
}

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" rel="stylesheet"/>


<div class="row">
  <div class="col-md-4">
    <div class="row">
    	<div class="col-md-4"></div> <!-- this column empty -->
    	<div class="col-md-4 cell"> 1 </div>
    	<div class="col-md-4 cell"> 2 </div>
    </div>
  </div>
  <div class="col-md-8">
    <div class="row">
    	<div class="col-md-2 cell"> 3 </div>
    	<div class="col-md-2 cell"> 4 </div>
    	<div class="col-md-2 cell"> 5 </div>
    	<div class="col-md-2 cell"> 6 </div>
    	<div class="col-md-2 cell"> 7 </div>
    	<div class="col-md-2"></div> <!-- this column empty -->
    </div>
  </div>
</div>

Well, IMO you probably need to override the width of the columns by using CSS3 @media query.

Here is my attempt to create a 7-col grid system:

<div class="container">
  <div class="row seven-cols">
    <div class="col-md-1">Col 1</div>
    <div class="col-md-1">Col 2</div>
    <div class="col-md-1">Col 3</div>
    <div class="col-md-1">Col 4</div>
    <div class="col-md-1">Col 5</div>
    <div class="col-md-1">Col 6</div>
    <div class="col-md-1">Col 7</div>
  </div>
</div>

@media (min-width: 768px){
  .seven-cols .col-md-1,
  .seven-cols .col-sm-1,
  .seven-cols .col-lg-1  {
    width: 100%;
    *width: 100%;
  }
}

@media (min-width: 992px) {
  .seven-cols .col-md-1,
  .seven-cols .col-sm-1,
  .seven-cols .col-lg-1 {
    width: 14.285714285714285714285714285714%;
    *width: 14.285714285714285714285714285714%;
  }
}

/**
 *  The following is not really needed in this case
 *  Only to demonstrate the usage of @media for large screens
 */    
@media (min-width: 1200px) {
  .seven-cols .col-md-1,
  .seven-cols .col-sm-1,
  .seven-cols .col-lg-1 {
    width: 14.285714285714285714285714285714%;
    *width: 14.285714285714285714285714285714%;
  }
}

The value of width comes from:

width = 100% / 7 column-number = 14.285714285714285714285714285714%

WORKING DEMO - (jsbin)

Run the code snippet and click on the "Full page".

.col-md-1 {
  background-color: gold;
}

@media (min-width: 768px){
  .seven-cols .col-md-1,
  .seven-cols .col-sm-1,
  .seven-cols .col-lg-1  {
    width: 100%;
    *width: 100%;
  }
}


@media (min-width: 992px) {
  .seven-cols .col-md-1,
  .seven-cols .col-sm-1,
  .seven-cols .col-lg-1 {
    width: 14.285714285714285714285714285714%;
    *width: 14.285714285714285714285714285714%;
  }
}


@media (min-width: 1200px) {
  .seven-cols .col-md-1,
  .seven-cols .col-sm-1,
  .seven-cols .col-lg-1 {
    width: 14.285714285714285714285714285714%;
    *width: 14.285714285714285714285714285714%;
  }
}

<link href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.4.1/css/bootstrap.min.css" rel="stylesheet"/>

<div class="container">
  <div class="row seven-cols">
    <div class="col-md-1">Col 1</div>
    <div class="col-md-1">Col 2</div>
    <div class="col-md-1">Col 3</div>
    <div class="col-md-1">Col 4</div>
    <div class="col-md-1">Col 5</div>
    <div class="col-md-1">Col 6</div>
    <div class="col-md-1">Col 7</div>
  </div>
</div>

Other options

Also, you could build your own 7-columns version of Twitter Bootstrap by using the Custom Builder (Changing the @grid-columns, ...).

If you are using less compiler, you could download the less version of Twitter Bootstrap (from Github) and edit the variables.less file instead.