Even though RNG does suck, here is a bit of probability.

To find out approximately how many FATEs you would have to do in order to be guaranteed an Atma (assuming the drop rate is 1%), first you have to find out the percentage chance of you NOT getting an Atma.

For example if you do 100 FATEs, the odds of you not getting an Atma is 0.99 (99%)^100 (or the number of FATEs attempted). This gives you 0.366. or a 36.6% chance of you not getting an Atma in the first 100 FATEs, or a 1 in 3 chance.

In order to reduce this chance of not getting an Atma close to 0, you would have to do 475 FATEs approximately per region to essentially be guaranteed an Atma (0.99^475 = 0.0084 or 0.8% chance). I certainly hope NO one is that unlucky to fall within that 0.8% chance, but if you guys want a basic number of FATEs you have to do, it will be 99.2% certain to be no more than 475 FATEs per region, or 5700 FATEs overall for all 12.

Now whether all that grinding is worth it is up to you.