<input type="text" maxlength="10" name="mobile" id="mobile" placeholder="Mobile"  title="<?php echo $this->__('Mobile') ?>" class="input-text required-entry validate-length maximum-length-10 minimum-length-10 validate-digits" />

if you want to use any number of integer validation on input filed use this 2 syntax.
maxlength=”10″
class=”input-text required-entry validate-length maximum-length-10 minimum-length-10 validate-digits”