The recommended amount of magnesium is 300mg to 500mg daily. As with calcium, chelated forms of magnesium are absorbed best by the body.
Here is a post which might help; it is a copy and paste of posters discussion of what magnesium works best for them in what amounts:
Magnesium:
https://www.inspire.com/groups/bone-health-and-osteoporosis/discussion/magnesium-9mp7me/
And a general one on bone supplements:
https://www.inspire.com/m/Kathleen1314/journal/7ed0c4-supplements-calcium-vit-k-collagen-etc/