JSFiddle - React, Tailwind, and code Playground

HTML

<div class="row">
    <div class="seat"></div>
    <div class="seat"></div>
    <div class="seat selled"></div>
</div>
<div class="row">
    <div class="seat selled"></div>
    <div class="seat"></div>
    <div class="seat selled"></div>
    <div class="seat selled"></div>
    <div class="seat"></div>
</div>
<button disabled="true">Нечего сохранять!</button>

CSS

div.row {
    height: 25px;
    margin: 10px 0;
    text-align: center;
}

div.seat {
    border; 1px solid #F00;
    background: #AAF;
    cursor: pointer;
    display: inline-block;
    height: 25px;
    width: 25px;
}

div.seat.selled {
    background-color: #FFA;
    cursor: default;
}

div.seat.selected {
    background-color: #FAA;
}

JavaScript

$$('div.seat:not(.selled)').addEvent('click', function() {
    this.toggleClass('selected');
    var selectedAmount = $$('div.seat.selected').length;
    if (selectedAmount != 0) {
        $$('button').set('disabled', false);
        $$('button').set('text', 'Сохранить(выбрано '+selectedAmount+')');
    } else {
        $$('button').set('disabled', true);
        $$('button').set('text', 'Нечего сохранять!');
    }
});