This page will show how the Dothraki number system works. As of yet we only have the names for the first ten numbers. These suggest that the Dothraki use a base ten number system. Higher order numbers are formed by fusing the unit number with the tens number etc. so the number eleven is formed by fusing the word for one, at, with the word for ten, thi, forming the word for eleven, atthi. Phonologically this is pronounced [aθ.θi] rather than [at.θi].
A large number like 2011 is written as "two thousand and eleven" which in Dothraki is akat dalen m'atthi.