Analysing Longitudinal Data - Aggregating waves

I am a novice at Stata and currently doing a project for my Econ undergrad econometrics module.

I am using data from UKDS Understanding Society: Longitudinal Data. Each observation is assigned with a personal id number representing an individual who was surveyed multiple times (waves) across many years. There is a maximum of 9 waves, but some individuals did not successfully complete all 9 surveys, some have only 1 or 4 waves of response etc.

Based on the summarize waves input, I found the average amount of waves responded to is 4.23 out of 9.

I want to be able to identify the personal id numbers which only completed less than the average amount of waves and drop them from the data. I do not want to merge the responses of each personal id number into one observation because this would distort the data too much.

What code can I use for stata to recognise each personal id number and put the ones that are the same together, such that I can output a count of how many waves each personal id completed?

I also want to be able to look at how each personal id number's income changed over time.