Have tried using PU_max(constant(), KPI), however the value changes with other dropdown selections/ filters. Is there any alternative way of generating constant value?

Have you tried using a static value variable? That seems to work for me. (Expand to see screenshots)

1. I grabbed a random dataset with 4281 cases.
2. Made a static variable which counted the number of cases (4281).
3. Made a component showing said variable.
4. Then applied a filter which reduced the number of relevant cases from 4281 to 2.

While the number up left shows 2 out of 4.28k, the static variable still shows the full 4281. Hope this helps!