The Wayback Machine - https://web.archive.org/web/20160617152122/https://dev.twitter.com/web/follow-button/parameters

Follow Button Parameter Reference

A follow button may be customized from its default settings by specifying parameter overrides.

screen_name required

The Twitter username to be followed.

Automatically extracted from the anchor element’s href attribute when using JavaScript-enhanced button markup.

Example Value: TwitterDev

show_screen_name optional

Set to false to hide the username of the specified account.

Example Value: false

show_count optional

Set to false to hide the number of accounts following the specified account.

Example Value: false

size optional

Set to large to display a larger button.

Example Value: large

lang optional

A supported Twitter language code.

Loads text components in the specified language.

Example Value: es

dnt optional

When set to true, the specified Twitter account does not influence Twitter targeting including suggested accounts.

Example Value: true

HTML example

<a class="twitter-follow-button"
  href="https://twitter.com/TwitterDev"
  data-show-screen-name="false"
  data-show-count="false"
  data-size="large"
>
Follow @TwitterDev
</a>

JavaScript example

twttr.widgets.createFollowButton(
  "TwitterDev",
  document.getElementById("container"),
  {
    showScreenName: "false",
    showCount: false,
  }
);

iframe example

<iframe
  src="https://platform.twitter.com/widgets/follow_button.html?screen_name=TwitterDev&show_screen_name=false&show_count=false&size=l"
  title="Follow TwitterDev on Twitter"
  width="80"
  height="30"
  style="border: 0; overflow: hidden;"
  ></iframe>