<form action="gonder.aspx" name="Form" method="post" onSubmit="return FormCheck();">
<input type="text" name="Text1" value="">
<input type="text" name="Text2" value="">
<input type="text" name="Text3" value="">
<input type="text" name="Text4" value="">
<input type="submit" name="Gönder" value="Gönder">
</form>

<script>
function FormCheck(){
TextCount = 4
MinCount = 2
iVal = 0
for (var i=1; i<TextCount ; i++) {
if(document.getElementById("Text"+i).value==""){
iVal = iVal + 1
}
}
if(iVal<MinCount){
alert("Lütfen en az "+MinCount+" alan doldurunuz.");
return false;
}else{
return true;
}
}
</script>