When creating a tag filter, Stack Exchange displays how often this particular tag is used at various sites. However, this number appears truncated due to long site names:
In the screenshot, the number appears truncated for most sites. On a closer look, it may be only the closing parenthesis that is truncated, but this is not immediately clear. Slightly more space needs to be reserved to display the number.
This is on Firefox 60.5.1esr on OpenSuse 15.0.