Use the given molarities!
You're dealing with a double replacement reaction in which two solutions that contain soluble ionic compounds are mixed together and form an insoluble ionic compound that precipitates out of solution.
In your case, a solution of cobalt(II) nitrate,
Now, when you're dealing with solutions, the problem usually provides you with their respective molarities.
#color(blue)("molarity" = "moles of solute"/"liters of solution")#
Since the two volume of the solutions are given, I assume that you were also given the two molarities of the initial solutions.
If that's the case, you need to use the above equation to solve for the number of moles of the solute
#c = n/V implies n = c * V#
So, for example, if the molarity of the cobalt(II) nitrate solution is
#n = 0.01"moles"/color(red)(cancel(color(black)("L"))) * 0.5 * 10^(-3)color(red)(cancel(color(black)("L"))) = 5 * 10^(-6)"moles Co"("NO"_3)_2#
You'd then go on to do the same for the trisodium phosphate solution.
So, as a conclusion, the problem must have provided you with the concentrations (molarities) of the two initial solutions - use them and the volumes to find the numbers of moles of each reactant you have.