Bit error rate (BER) is a key indicator of asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM). The number of subcarriers is an important factor of the indicator. By theoretical deduction and simulation, the impact of subcarriers′ number on the BER of ACO-OFDM transmitted in standard single-mode fiber (SSMF) with optical single-sideband (OSSB) modulation is analyzed. Theoretical analysis shows that, when the total transmission rate is constant, changing the number of subcarriers from 128 to 16384, the total bandwidth required changes no more than 0.775%. And based on this, further theoretical analysis and simulation illustrate that, with different transmission rates of ACO-OFDM system, there always exists the optimal number of subcarriers resulting the lowest BER, and the optimal number increases with higher transmission rate.