From the article:
Mastodon is a great microblogging platform, but maybe you want to try something new. Firefish made a splash recently, and a lot of people are talking about it. If you’ve played with it for a while and want to officially take the plunge, here’s some tips on moving from your Mastodon Instance to a Firefish one.
AFAIK mastodon has a hard limit of 500, changing it would either require source code modding, usage of a fork or using another software
Well, I know there are Mastodon servers that allow more than 500. Guess they did have to modify the code or run a fork though.