"Making up the top one head" is an idiom used to describe an estimate or a rough guess of a particular number or quantity. However, there are several other synonyms for this phrase that can be used interchangeably such as "off the top of one's head", "ballparking", "guesstimate", or "rough calculation". These synonyms indicate that the speaker is not entirely sure of the exact number or quantity but is providing an estimate to the best of their knowledge. Therefore, it is important to consider the context in which these phrases are used to accurately interpret the intended meaning.