There are several ways that you can do so:
1) use the "Count the reciprocal" method: How to count the number of unique occurences in field.
2) Define a new table with a field for the company name. Specify a validation field option for this field as: Unique values, validate always. Import all your records from your existing table into this new table using Import records and enable "auto-enter" options in the check box for that which appears in a small dialog that pops up just before performing the Import. The validation rule will omit duplicate values during the import so the resulting number of records in this table will be the number of different companies in your table.
You may find that such a table with one record for each company very useful for your database and once you have set up the initial table from your current data, there are a number of fairly simple ways to keep it up to date.
3) Execute SQL can can count the number of unique companies if you are using FileMaker 12: A new way to count unique values in FileMaker 12