The standard amount of glucosamine and chondroitin that should be given to your dog is 20mg's per pound of weight, so in the case of your 20lb dog the amount of glucosamine and chondroitin would be 500mg's.
You should try getting a supplement that also contains a natural anti-inflammatory such as MSM alongside glucosamine and chondroitin to help ease the pain and swelling that so often occurs with joint pain in dogs.
A dog should not be having amoxacillon; Check with your vet to see what medicines your dog might need