You can follow a hadith in al-Kafi if you know the meaning of the hadith, you have understood it well, and you are aware of its context and the other hadiths that shed light on it. If it’s a moral/ethical teaching you can follow it usually without a concern. If it’s a legal ruling, you need to be an expert yourself or consult an expert like a Mujtahid to follow it

There is no set number for how many Sahih hadiths there are in al-Kafi. Different scholars have different standards for that. However, historically most scholars have accepted most of Al-Kafi.