k=0
for(i=0;i<form1.all('test').length;i++)
{
if(from1.all('test')[i].checked){k+=1}
}
if(k>=2){alert('OK')}
for(i=0;i<form1.all('test').length;i++)
{
if(from1.all('test')[i].checked){k+=1}
}
if(k>=2){alert('OK')}
<script>
function check(uid)
{
j=0;
for(i=0;i<form1.test.length;i++)
{
if(form1.test[i].checked)
j++;
if(j>1)break;
}
if(j>1)
form1.submit();
else
alert("more than 2 checkboxs will be checked");
}
</script>
<form action="debug" name="form1" method="post">
<input type="checkbox" name="test" value="sdf1">
<input type="checkbox" name="test" value="sdf2">
<input type="checkbox" name="test" value="sdf3">
<input type="checkbox" name="test" value="sdf4">
<input type="button" value="submit" onclick=check()>
</form>