reachable users question

Total = all users in your app group with the filters defined above.
Email = opted in or subscribed
iOS/Android push = users who have a push token and their push subscription status = subscribed/opted in

These are approximations. For the actual numbers, you can click Calculate exact statistics.
When you want to calculate exact stats in a specific segment (= not in a campaign), you have to do it after each filter because the calculation will only be correct after the last filter. Everything before will be an approximation.