How often can I safely take Senna, & at what dosage. 2 tablets at bedtime did not work?

Have you had a problem with constipation in the past or has this just happened?
If this is happening regularly you may need to look at the cause and how you can help yourself eg. increase fiber in diet and exercise and drink plenty of fluids..
Read more : https://www.drugs.com/health-guide/constipation-and-impaction.html

If this is a one off problem you can take 2 to 4 Senna tablets at night as required. It is not safe to take senna each night for on going relief.
If you feel you need to take something regularly talk to your doctor or pharmacist about a safer long term medicines such as Metamucil (psyllium).