To set a container for Bootstrap card and set it like a group, use the card-group class.
Use it and create a grid like the following with two Bootstrap cards:
<div class="card-group">
<div class="card bg-primary">
<div class="card-body text-center">
<p class="card-text">Demo Text!</p>
</div>
</div>
<div class="card bg-warning">
<div class="card-body text-center text-white">
<p class="card-text">Warning!</p>
</div>
</div>
</div>
The following is an example to create a grid (group of cards) in Bootstrap:
Live Demo
<!DOCTYPE html>
<html lang="en">
<head>
<title>Bootstrap Example</title>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
</head>
<body>
<div class="container">
<h3>Group of Messages</h3>
<div class="card-group">
<div class="card bg-primary">
<div class="card-body text-center">
<p class="card-text">Demo Text!</p>
</div>
</div>
<div class="card bg-warning">
<div class="card-body text-center text-white">
<p class="card-text">Warning!</p>
</div>
</div>
<div class="card bg-success">
<div class="card-body text-center">
<p class="card-text">Well done!</p>
</div>
</div>
<div class="card bg-success">
<div class="card-body text-center">
<p class="card-text">Selected!</p>
</div>
</div>
</div>
</div>
</body>
</html>