Hopefully you’ve been nice to your professors and bosses, since you’re going to need several of them to write letters of recommendation for you.
Check each application to see how many letters are required, but you generally need between two and four. If at all possible, the majority should come from academic references.
If you’ve been out of school for a while, it’s fine to include a letter from an employer, along with the academic letters, but try not to rely solely on employer recommendations.