Based on the information available from DrugPatentWatch.com, the top manufacturers of tamslulosin, a medication used to treat an enlarged prostate, in 2020 were:

1. Astellas Pharma: Astellas Pharma, a Japanese pharmaceutical company, is one of the leading manufacturers of tamslulosin. Their product, Flomax, is a commonly prescribed medication for the treatment of an enlarged prostate.

2. Dr. Reddy's Laboratories: Dr. Reddy's Laboratories, an Indian multinational pharmaceutical company, is another major manufacturer of tamslulosin. They produce a generic version of the medication.

3. Mylan: Mylan, an American pharmaceutical company, also manufactures a generic version of tamslulosin. They are one of the top manufacturers of the medication.

4. Teva Pharmaceutical Industries: Teva, an Israeli multinational pharmaceutical company, is a leading manufacturer of generic drugs, including tamslulosin.

5. Cipla: Cipla, an Indian multinational pharmaceutical and biotechnology company, is another significant manufacturer of tamslulosin. They produce a generic version of the medication.

Please note that the ranking of these manufacturers can vary based on factors such as market share, sales, and production volume.

Sources:
[1] DrugPatentWatch.com, "Tamsulosin Hydrochloride", https://www.drugpatentwatch.com/drugs/tamsulosin-hydrochloride.