In that case: the instruments in an orchestra don't have to play individual lines. If you are familiar with synth programming, you can think of the orchestra groups like they are wavetable oscillators. By giving the same notes to different instruments, you are designing the sound you are creating. You can build a full and epic orchestra sound by mostly copy/pasting phrases to other instruments (maybe shift it up or down an octave, as long as that does not get in the way of the rest of the arrangement).
Arrangement: Make sure you don't have several melody lines in the same pitch range. If you have some counterpoint or similar going on, it helps if they are not trying to inhabit the same frequency space. And since you are limited by how many octaves you have, you are automatically limited in the number of melody lines you can run at the same time ;) (I am currently in the process of learning how to cut down on lines for clarity)
Also, I'm a hobbyist without a music or composition degree, so feel free to ignore anything I'm saying that sounds like bs to you.