The adverb mal is the colloquial form of einmal and it means once. But
That's want I once call a coincidence.
doesn't make much sense.
This is, because in »Das nenn ich mal einen Zufall« the word mal is not an adverb!
It is a modal particle. English Wikipedia has a very good article about those beasts (the German version is even more detailed)). German (especially colloquial german) is full of modal particles, and they hide in plain sight, because every modal particle is a homonym of another word that belongs to another part of speech.
Modal particles are very often used in colloquial German, but the do not exist in most other languages. English is a language without modal particles. This means: There are no direct translations for those words. There simply are no counterparts for German modal particles in English language.
What do they mean?
They mean nothing. (In more scientific terms: Modal particles do not add anything to the proposition of a sentence.) They just add a vague emotion to the sentence. And this is the reason why they are so hard to translate into other languages. In any other language than German you have to describe the emotion in longer phrases. German has simple words for this purpose. The simplest way to deal with them if you want to translate them into english is to ignore them.
So, when you translate a German sentence containing modal particles into English, and translate this English sentence back to German, in most cases you wil get a German sentence without those modal particles.
So, the best way to translate the sentence is this:
Das nenn' ich mal einen Zufall.
English translation: (two versions)
1. I call this a coincidence. (literal translation, but low quality style)
2. That's what I call a coincidence. (free translation, but better style)
Back to German: (three versions)
1. Das ist, was ich einen Zufall nenne. (matches with Engl. #2, but its bad style in german)
2. Ich nenne das einen Zufall. (matches with Engl. #1, but sounds boring)
3. Das nenne ich einen Zufall. (also matches with Engl. #1, but has a different word order which is much better style in german)
As you can see, there is no mal in the German back-translation. Like all modal particles it became lost in translation.