I'll teach you how to count how many times a value appears in a vector. Use the count function from algorithm with int freq = count(v.begin(), v.end(), target). This scans the entire vector and returns the number of matches.
For multiple targets, loop through each one and call count. If you need frequencies of all unique values, use a map instead. To count elements that match a condition like even numbers, use count_if(v.begin(), v.end(), [](int x) { return x % 2 == 0; }) with a lambda function that returns true for matches.