How can I insert multiple rows into database in Laravel?

You are always using the same instance of the object. Each time you save the same row with different data. here is how to fix.

$num = 5;

for ( $i = $num; $i > 0; $i-- ) {

    $voucher = new Voucher;

    $token = generateRandomString(5, ['number', 'uppercase']);

    $voucher->prefix = $request->prefix;
    $voucher->token = $token;
    $voucher->description = $request->description;
    $voucher->date_time = time();
    $voucher->save();
}

You should create a new voucher within the for-loop. Currently, as you said, you are inserting the same object into the database five times

You are using same instance everytime! Create new one in every for loop like:

$num = 5;

for ( $i = $num; $i > 0; $i-- ) {

    $voucher = new Voucher;

    $token = generateRandomString(5, ['number', 'uppercase']);

    $voucher->prefix = $request->prefix;
    $voucher->token = $token;
    $voucher->description = $request->description;
    $voucher->date_time = time();
    $voucher->save();
}

This is basic example create multi array and simply put in insert function.

$num = 5;
$dataArray = array();
for ( $i = $num; $i > 0; $i-- ) {

    $token = generateRandomString(5, ['number', 'uppercase']);

    $dataArray[$i]['prefix'] = $request->prefix;
    $dataArray[$i]['token'] = $token;
    $dataArray[$i]['description'] = $request->description;
    $dataArray[$i]['date_time'] = time();

}

Voucher::insert($dataArray); 

If you already have a model instance, you may use the fill method to populate it with an array of attributes:

$num = 5;
$voucher = new Voucher;

for ( $i = $num; $i > 0; $i-- ) {

    $token = generateRandomString(5, ['number', 'uppercase']);

    $payload = [];
    $payload['prefix'] = $request->prefix;
    $payload['token'] = $token;
    $payload['description'] = $request->description;
    $payload['date_time'] = time();

    $voucher->fill($payload);
}

Documentation