Number of elements in float array in C

i have array of float in C. the length of this array depends of user input. I need to find out number of elements in this array. Example:

float a[10] = {3.1314, 1.5131, 9.133,  10.333}

How can i check number of elements? sizeof(a)/sizeof(a[0]) always give the max number of elements (10 in this case).