Block insertion in box

Question

In the code I create two box, one patrimonio and another serial:

<table border="0" align="center" height="100">
    <tr>
        <td>   
            <font face="arial" align="center" valign="middle" color="blue" size="-1">PATRIMÔNIO</font><br>
            <input type="text" name="tx_patr" id="id_patr" maxlength="12" size="12" style="font-size:11; color:Black;" onkeypress="return SomenteNumero(event);" onkeyup="Mascara(this,Patri);" value="">
            <input type="button" onClick="move_patr_seri(this.form.tx_patr,this.form.cb_Patr);limpa_patr();" value=">>">
            <br>
            <select multiple size="7" name="cb_Patr" style="width:300"></select>
            <br>
            <input type="button" align="center" valign="middle" onClick="tira(this.form.cb_Patr)" value="<<">
            <br>
        </td>
        <td align="center" valign="middle" width="15%">
        </td>
        <td>  
            <font face="arial" align="center" valign="middle" color="blue" size="-1">SERIAL</font><br>
            <input type="text" name="tx_seri" id="id_seri"  maxlength="15" size="15" style="font-size:11; color:Black;" onblur="evento(this)"  value="">
            <input type="button"  onClick="move_patr_seri(this.form.tx_seri,this.form.cb_Seri);limpa_seri();" value=">>">
            <br>
            <select multiple size="7" name="cb_Seri" style="width:300"></select>
            <br>
            <input type="button" align="center" valign="middle" onClick="tira(this.form.cb_Seri)" value="<<">
            <br>
        </td>
    </tr>
</table>  

I want to block a box insertion if an informed amount (coming from a previous program) is reached. For example: "Informed quantity = 3, so the user can only enter 3 assets and if he wants 3 serials. If you want to enter 4 assets or serial it must be blocked."!

EXAMPLE IN JSF: Example of the box and the insertion Javascript

Obs* The variables that are in JSF

var w_Qtde_Peri = 3; -> Quantidade informada
var w_Cont_Qtde = 0;
     var  w_ver = 1;
     var v_patr = new Array (w_Qtde_Peri);  

are of PHP, so it will not include in the box!

Answer 1

I managed to come up with an answer to my problem!

In case to block, just need to add this snippet:

 if((Destino.name == "cb_Seri") && (Destino.options.length == w_cont))
    { return false;} 
 if((Destino.name == "cb_Patr") && (Destino.options.length == w_cont))
    { return false;} 

Where w_cont receives the informed quantity, thus not allowing the user to enter more than the total quantity in the box!