Is there a relation between dehydration and RBC count in urine?

Usually no. However, if you have a few red cells in urine, a concentrated urine in dehydration will have less water and although the count of the cells being discharged has not changed, the count per a unit volume, eg cc or cmm, may increase. Normally ther should be no blood in urine. If you have blood in urine, confirmed by repeat, need to see a urologist.