California housing prices dataset on Kaggle
It looks like the dataset is aggregated from another raw dataset, e.g. the mean house price has been calculated from the raw data listing every house individually.
The creator of the dataset perhaps made a mistake, or intentionally left out the number of houses, perhaps not thinking this feature was relevant for the use-case in mind.
Either way, the dataset is made from a census taken in California in 1990, so perhaps you need to search for the raw census data instead and calculate it yourself.