how many smaller circles(radius is equal) I can fit within a larger circle then the question is,the larger radius D,the small radius d,get the largest number of small circle put in the larger?
The answer can be closely approximated by this equation:

Number of Circles $= 0.83\frac{R_2^2}{r_1^2} - 1.9$  (rounded down to whole number)

where: $R_2$ = radius of larger circle
       $r_1$ = radius of smaller circle
The answer can be closely approximated by this equation:

Number of Circles $= 0.83\frac{R_2^2}{r_1^2} - 1.9$  (rounded down to whole number)

where: $R_2$ = radius of larger circle
       $r_1$ = radius of smaller circle