I think in this case there isn't an exact translation. It's supposed to be "I have tens (in the 11-19 range) of plates" would be more accurate for this sentence since belasan are the ones that have -belas, as the previous commenter said. But since 11-19 in English is included with 20 and above, there is no translation.
In Indonesian, a number estimation will always be the ending word (I don't know what it's called sorry) with -an. For example ratusan, ribuan, jutaan, milliaran, and so on.
So the point is, selusin will always mean 12 (a dozen) exact, while belasan can be any in the 11-19 range.