There are no R Signatures. The R series are General Production reservation holders that have previously purchased Roadsters.
The total number of Special Signatures and Regular Signatures is 1,000. Yes, the last Signature will get the 1000th car.Maybe Tesla is doing it like this: Ex. S1, SSL2, R3, R4, SSL5, etc...to 1,000 cars. S1278 is going to get S1000 accounting for people that deferred/cancelled
The General Production starts with the R series then goes to the regular General Production series.