That is wonderful!

My guess is 2.

Doing some quick reading, there looks to be a surprising amount of studies on Ural Owl clutch sizes. Here is one with some nice graphs.

The studies seem to revolve around the vole cycle, which they say occurs in 3 year periods. When more prey is available, females tend to lay earlier (early March-ish?) and will have more babies (4 is the normal high end number), while later laying (they listed March 31 as the 0 on their X axis) the number reduces to where 2 is the norm.

The paper I linked is from the 80s, so I'm not sure how much your weather has changed since then and how that has affected things.

You may be the first of us to have your own baby owl guests! I wish your owl friends the best of luck and successful hunting to get those babies grown up big and strong, and I hope you get to see them fledge and branch and all that fun stuff!

Thanks for the updates!