Removing double values from a column in R

I need to know how many different items are produced by each company (ID). The result should be this:

• Company 1 = 4 (A,B,C,D)
• Company 2 = 1 (B)
• Company 3 = 2 (A,B)
• Company 4 = 2 (A,B)

Which code should I use to get there? I guess it should be something that allows me to count the unique values for each ID.

Thanks