You need to evaluate the stability of your suspension. In fact, if the suspension is stable (no precipitation) you can add more.
Then, the answer of your question is mainly depends on your filler and polymer characteristics as well as your solvent. At the end, to much filler may also negatively affect the finer formation during electrospining.
As Ebrahim mentioned, the process and solution stability will set the maximum possible amount. Up to 80% and maybe even more 2nd phase is possible. Also to be considered price of 2nd phase and the desired functionality and application of your product
my be the best amount of second phase leads to high dispersion a nanoparticles in polymeric solution and produced the goodness results in morphological , mechanical and crystalline properties .