We have known for a while that Russia bought from Ukraine about 30 SS-19/UR-100NUTTH missiles, which it intends to keep in service (although there are some questions about continuing flight tests of the missile). But the exact number of these missiles has never been officially disclosed. It still hasn't, but we have an interesting data point - a report in the VPK newspaper mentions that in 2003 the Russian armed forces received 26 missiles of this type.
Update: As it turned out, there are some numbers on the Ukrainian missiles. It looks like 29 of them were transferred to Russia. Which already was three missiles fewer than Ukraine declared in START. Now, if we believe the VPK number (although, I must admit, there is no particular reason to), three more are unaccounted for.