Limita scelta Checkbox, Script Gestione campi Form

<script type="text/javascript">
function disabilita(_id){
var lista_check = document.getElementsByTagName("input");
var conta = 0;
for(i = 0; i < lista_check.length; i++){
if(lista_check.item(i).getAttribute("type") == "checkbox" && lista_check.item(i).checked == true){
conta++;
}
}
if(conta > 5){
document.getElementById(""+_id).checked = false;
}
}
</script>
<br>
Massimo 5 selezioni
<form name="mioform">
1: <input name="nome" id="check1" type="checkbox" onclick="disabilita(this.id)"/>
2: <input name="nome" id="check2" type="checkbox" onclick="disabilita(this.id)"/>
3: <input name="nome" id="check3" type="checkbox" onclick="disabilita(this.id)"/>
4: <input name="nome" id="check4" type="checkbox" onclick="disabilita(this.id)"/>
5: <input name="nome" id="check5" type="checkbox" onclick="disabilita(this.id)"/>
6: <input name="nome" id="check6" type="checkbox" onclick="disabilita(this.id)"/>
7: <input name="nome" id="check7" type="checkbox" onclick="disabilita(this.id)"/>
8: <input name="nome" id="check8" type="checkbox" onclick="disabilita(this.id)"/>
<input type="button" value="Clicca">
</form>