"Too many" to me would be if you are purchasing additional mods at the expense of neglecting other items that your family may want or need. That's the practical answer.
Are you a hobbyist or collector? I've seen members here who literally have hundreds of mods in their collection. They are collectors, they are not collecting mods for the purpose of having "backups for their backups". Some people spend hundreds/thousands of dollars on artwork, vacations, clothing, cars, guitars, etc. Seems excessive to me, but who am I to judge how other folks spend their hard earned money?
I have over a dozen mods myself. All collected over a 6 year period of time. Most of those have been "retired" as their usefullness has been surpassed by a couple of favorite mods, but they still hold some sentimental value to me. I could easily get by with just two mods.