Answer:
The number they would count is [tex]\approx 0.49\cdot10^{-2}\ \%}[/tex]
Step-by-step explanation:
One mole is equal to 6.02214076x10^23 particles. In order to know the number counted by every human on Earth we simply multiply the number of humans times the count a "good counter" can achieve, 4x10^9x7.44x10^9 (for USA, change to 4x10^12 if you use de British English definition). Thus:
[tex]\displaystyle{\frac{4\cdot10^9\cdot7.44\cdot10^9}{6.02214076\cdot10^{23}}=\frac{2.976\cdot 10^{19}}{6.02214076\cdot10^{23}}\approx 0.49 \cdot 10^{-4}\approx 0.49\cdot10^{-2}\ \%}[/tex]