"I have ten different companies."
Terjemahan:Saya memiliki sepuluh perusahaan berbeda.
I have almost always seen companies translated as "perusahaan". In fact I'm having trouble thinking of a time I've ever seen or heard firma be used. So while it may technically be correct, perusahaan is the better translation
There are firma, perseroan, perseroan terbatas, and they are all perusahaan. That's why it's more common. It will be easier to translate firm as firma and company as perusahaan.