HTML

accept only numbers in textbox

<input type="text" onkeypress="return isNumberKey(event)" placeholder="Phone Number">
//this will accept only numbers from 0 to 9
<script>
function isNumberKey(evt)
{
	var charCode = (evt.which) ? evt.which : event.keyCode
	if (charCode > 31 && (charCode < 48 || charCode > 57))
	return false;

	return true;
}
</script>

input only accept numbers

<label for="salary">Enter your salary:</label>
<input type="text" id="salary" name="salary"
    oninput="this.value = this.value.replace(/[^0-9.]/g, '').replace(/(..*)./g, '$1');" />

accept limited numbers in text input

<!-- You can put any number you want in maxlength i.e 4 -->
<input type="text" maxlength="4" onkeypress='return event.charCode >= 48 && event.charCode <= 57'/>

input only accept numbers

  <input type="number" name="age" id="formAge" maxlength="3" placeholder="age">