In order to count Tweets in real-time, you will need to use the Twitter Streaming APIs, in particular the statuses/filter
endpoint. Using the track
parameter with the hashtag you are interested in, you will receive matching Tweets in real-time.
In order to get started, you will need to create credentials for your Twitter app on https://apps.twitter.com/, then consume the API from your favorite language. The fastest way is to use an existing library which will handle the authentication and the connection to the Twitter API for you.
If you want to use Go, I definitely recommend twittergo by @kurrik which supports the Streaming APIs. For other languages, you can take a look at this page on Twitter Libraries.