Convert Concentration (particles/ml) to (mg/ml)
First lets see how to get from particles $N$ per volume to mass $m$ per volume.

Dividing the particle number by the Avogadro constant gives moles.

$$
\frac{N}{V}\cdot \frac{1}{N_\mathrm A}=\frac{n}{V}
$$

Multiplying moles with molar mass gives mass.
$$
\frac{n}{V}\cdot M=\frac{m}{V}
$$
Concluding:
$$
\frac{N}{V}\cdot \frac{M}{N_\mathrm A}=\frac{m}{V}
$$

Now your problem arises because you do not know the molar mass. Because you probably have some distribution function of particle size and not just one size. I just assume now that you made an emulsion polymerization in order to get the latex particles or bought them commercially. Then this distribution is very sharp and you can easily take the averaged molar mass as the molar mass of the most probable particle size.