Automatic number for form

I wanted to get an automatic number after completing a form, or, I suppose, I'll fill out my form. What you have to say, Reserve # 1/2018, when someone else has to fill in another form, you have to say Reserve # 2/2018. I do not know if you can understand. Thanks to all who can help me.

My html:

Reservation No. / 2018

    <tr>
        <td>
            Dia:
        </td>
        <td>
            <input name="dia" type="date"  required />
        </td>
    </tr>

    <tr>
        <td>
            Pessoas:
        </td>
            <td>
             <input name="pessoas"  type="number"  MAX="14" required />
            </td>
    </tr>

    <tr>
        <td>
            Hora:
        </td>
            <td>
            <select name="hora"type="text" required >
                <option value="0"></option>
                <option value="12:30">12:30</option>
                <option value="13:00">13:00</option>
                <option value="13:30">13:30</option>
                <option value="14:00">14:00</option>
                <option value="14:30">14:30</option>
                <option value="15:00">15:00</option>
                <option value="15:30">15:30</option>
                <option value="18:30">18:30</option>
                <option value="19:00">19:00</option>
                <option value="19:30">19:30</option>
                <option value="20:00">20:00</option>
                <option value="20:30">20:30</option>
                <option value="21:00">21:00</option>
                <option value="21:30">21:30</option>
                <option value="22:00">22:00</option>
                <option value="22:30">22:30</option>
                <option value="23:00">23:00</option>


            </select>
            </td>
    </tr>


    <tr>
        <td>
            Nome:
        </td>
            <td>
                <input name="nome" type="text" placeholder="Nome Completo" required />
            </td>
    </tr>



    <tr>
        <td>
            Numero:
        </td>
            <td>
                <input name="numero" type="number" required" />
            </td>
    </tr>


    <tr>
        <td>
            E-mail:
        </td>
            <td>
                 <input name= "email" type="text" required />
            </td>
    </tr>


                        <tr>
        <td>
        Mesa:
        </td>
            <td>
                <input name="mesa" type="Number" value="Reservar" max="11" required />
            </td>
    </tr>

    <tr>
        <td>    
        </td>
            <td>
            <div class="butoesdelembrar">
              <input type="checkbox" checked="checked" name="remember" style="margin-bottom:15px font-size:60;"> Receber informação de eventos futuros?<br>
            <input type="checkbox" checked="checked" name="remember" style="margin-bottom:15px"> Promoções e atualizações deste restaurante<br>