In order to keep blood pH at 7.4, the HCO3: H2CO3 ratio should be:

lt is evident that at a blood pH 7.4, the ratio of bicarbonate to carbonic acid is 20:1. Thus, the bicarbonate concentration is much higher (20 times) than carbonic acid in the blood. This is referred to as alkali reserve and is responsible for the effective buffering of H+ ions, generated in  the  body. In normal circumstances, the concentration of bicarbonate and carbonic acid determines the pH of blood. Further, the bicarbonate buffer system serves as an index to understand the disturbances in the acid-base balance of the body. Satyanarayana, 3rd Edition, Pg No. 476