You need at least two numbers to find something in common.
This cannot be determined because the common multiples of any two or more numbers are infinite.
Common Multiples of 8:8,16,24,32,40,48,56,64,72,80,88 and 96 Common Multiples of 12:12,24,36,48,60,72,84 and 96
12 and any of its multiples
Since 36 is a multiple of 12, all of its multiples are common.
Since the LCM is 12 then the common multiples are multiples of 12 12, 24, 36, 48
Least common multiples 12 and 16 is 48.
The first 3 common multiples of 4 and 12 are: 12, 24, 36
The first 2 common multiples of 4 and 12 are 12 and 24.
Multiples of 12 are 12, 24, 36, 48, 60, 72, 84... Multiples of 42 are 42, 84... Two common multiples of 12 and 42 are 84 and 168. The Least Common Multiple of 12 and 42 is 84.
A single number cannot have a highest common multiple because "common" refers to multiples that two or more numbers have in common. You have only one number.
21
There ain't no such animal! Highest goes with Common FACTORS; LEAST goes with Common Multiples.