Did you try -o read -n 100000? The issue isn't that you were trying to read with a different n, it's that you were trying to read with an n which has a different order of magnitude. When trying to read -n 1, see that it was trying to read key 0? That key doesn't exist from a -n 100000 insert, since it will have only inserted keys 000000-099999.
To explain it more generally and without so many confusing sets of zeroes...
If you go into Data Explorer in OpsCenter, or simply do a list CF in cassandra-cli, you'll see that each key generated by a -o insert -n x will have the same number of digits as x does. This is the problem you're running into when trying to -o read -n y. If y doesn't have the same number of digits as x, the read will fail as it's trying to access keys that don't exist.
In general, you'll want to only use the stress -o read with the same -n you used during the insert. The farthest you can deviate from this would be to -o insert -n 9999 and then -o read -n 1000 (And similar 9...9->100...0 patterns), since they have the same number of digits they will both be accessing the same keys.