if we have an RNA transcription and we add an additonal temination site in the middle , will i have two RNA molecules or just a short one ?
Assuming that by termination signal you mean transcription termination signal (and not a stop codon), you would have one shorter RNA molecule. You can have two shorter RNA molecules if there is another transcription start site after the termination signal, but there is no reason to assume this will be the case.
In some cases the RNA polymerase will skip a base and frameshift while transcribing a new RNA strand. This is known as transcriptional slippage, and it is taken advantage of by some viruses in order to make their genomes more compact by having several different mRNAs overlap each other.
In this rare case, having a termination signal does not necessarily mean transcription will terminate, it might be able to skip through parts of the termination signal and continue to the next termination site.