According to LiveScience, the volume of seawater in the oceans is about $1.3 \times 10^8$ $km^3$, or $1.3 \times 10^{17}$ $m^3$, or $1.3 \times 10^{20}$ liters, or $5.6 \times 10^{20}$ cups.

A mole of water is $18$ $gm$, which is $18$ $cm^3$. A cup is $237$ $cm^3$ or $13.2$ moles.

A mole is also $6 \times 10^{23}$ molecules of water, so a cup is $7.9 \times 10^{24}$ molecules.

If you mix $7.9 \times 10^{24}$ labelled molecules in $5.6 \times 10^{20}$ cups of seawater, each cup will contain about $14,000$ labelled molecules.

Unless I have made a silly arithmetic mistake.

According to LiveScience, the volume of seawater in the oceans is about $1.33 \times 10^9$ $km^3$, or $1.33 \times 10^{18}$ $m^3$, or $1.33 \times 10^{21}$ liters, or $5.6 \times 10^{21}$ cups.

A mole of water is $18$ $gm$, which is $18$ $cm^3$. A cup is $237$ $cm^3$ or $13.2$ moles.

A mole is also $6 \times 10^{23}$ molecules of water, so a cup is $7.9 \times 10^{24}$ molecules.

If you mix $7.9 \times 10^{24}$ labelled molecules in $5.6 \times 10^{21}$ cups of seawater, each cup will contain about $1400$ labelled molecules.

Unless I have made a silly arithmetic mistake.