Currently you can only have one automatic discount per product. You can, if you want, create the other levels with simple codes, so no discount for 1-50, then automatic for 51-100 and then a code called tier 3, or something like that, for 101-150 and so forth.
The other option is as you stated the variant feature. This would take more time, as you would need to setup each level completely, like this
1 - $.85
2 - $1.70
3 - $2.55
...
...
51 - $41.82
52 - $42.64
And so forth. It will take a lot more setup, but it will be more automatic for your buyers.
