While this is not possible to do via the event settings tab, you can make this adjustment by editing the HTML code in the event page's template section. There is a line of code in the template for that page that says this:

<div class="span12"><label for="event_rsvp_guests_count">How many other people are you bringing?</label>{% text_field "guests_count", class:"text" %}</div>

But you could convert it to all HTML, change it from a text field to a number field, and then apply the "max" attribute to that field. The resulting code would look like this instead:

<div class="span12"><label for="event_rsvp_guests_count">How many other people are you bringing?</label><input style="width:50px;" max="3" id="event_rsvp_guests_count" name="event_rsvp[guests_count]" type="number" value="0" /></div>

That should do it! 👍

Here are some additional attributes that can be added to <input> fields:

http://www.w3schools.com/html/html_form_attributes.asp