Many individuals do not get enough calcium in their diets alone, especially as they get older and absorb calcium less well. Most people who take precautions against sunburn and skin cancer do not get enough vitamin D. Vitamin D needs increase as we get older, naturally making and utilizing vitamin D less well.
We have over 100 posts on these subjects under the Calcium and Vitamin D categories. Some specifically address the problems of Calcium Carbonate absorption in some individuals and recommend calcium citrate or another readily absorbable calcium supplement.
NOF has an excellent summary at https://www.nof.org/calcium-vitamin-d-supplement-facts/.
The NOF recommendations are for maintenance of good bone health. Those with fractures and/or osteoporosis by DXA/VFA/TBS often need to play catch-up and require higher doses.
A complete bone health evaluation and discussion with your health care professional can individualize your situation.
Jay Ginther, MD