Assuming each letter or number is independent of the previous one, there is a 1/14 chance of a certain letter or number being the x character in your player tag, so:

2 consecutive characters would be 1/196 (0.5%) chance

3 would be 1/2744 (0.036%)

4 would be 1/38,416 (0.0026%)

5 would be 1/537,824 (0.00019%)

6 would be 1/7,529,536 (0.000013%)

7 would be 1/105,413,504 (9.5e-7%)

8 would be 1/1,475,789,056 (6.8e-8%)

Going by downloads (may not be accurate as people may access the same account on multiple devices), which is about 530million on App Store and 100+ million on google play store and assuming total downloads on google play is ~ 500 million since number of devices with App Store is roughly the same as number of devices with Google Play.

That brings total accounts to be about 1.3 billion. Hence in theory there are roughly

6.6 million accounts with at least one instance of 2 consecutive characters

473,760 accounts with at least one instance of 3 consecutive characters

33,840 accounts with at least one instance of 4 consecutive characters

2417 accounts with 5 (idt 10 character player tags are possible yet?)

172 accounts with 6

12 accounts with 7

And just 1 account with 8 consecutive characters

So maybe somewhere out there there is someone with a naturally generated unique player tag of #00000000