Measuring in number may be misleading as it depend on the size of the local authority. Maybe an indicator based on Yes/No related to the involvement of such professional in the process of designing services can be more significant

Good indicator, would advise two others as well: 1. Percentage of professionals with UX experience on the total population (huge city with one UX specialist vs small village with one UX specialist) 2. Try to determine the quality of the people working on/with UX, the existence of (what number / percentage of) processes in which UX is mentioned. In the latter it of course matters whether or not the UX is executed during the process

In addition to the number there could be extra question whether in developing new services people with such expertise are involved.

also referring to the questions/suggestions above: what will be the (objevctive) indicator? who counts as "professional with skills"? will I? no diploma's, just experience. do we need a questionaire to assess these things?

It also depends a lot on the city's service creation strategy. An IT department that completely outsources the creation of services is not the same as one that creates them in-house.

In addition to this indicator is important to know the number of professionals than works into business units, who knows about it, and do not work in IT department. May be they are "IT embassadors"