How to do this ?

add attribute max="10"

Where to add ?

<div class="row">
              <label for="phoneMobile"><span><?php _e('Mobile phone', 'veronika'); ?></span><span class="req">*</span></label>
              <div class="input-box"><i class="fa fa-mobile"></i><?php UserForm::mobile_text(osc_user()); ?></div>
            </div>

into this function:
UserForm::mobile_text

Not working