From this NASA FAQ page about Europa:
Theory and observation indicate that Europa's icy shell is around 15 to 25 kilometers (10 to 15 miles) thick, overlying an ocean approximately 60-150 kilometers (40 to 100 miles) deep. Support for this hypothesis comes from observations of pits, domes, and spots on Europa's surface. The size and spacing of the features suggests that they are due to churning within the ice shell, and theory suggests that such churning (called convection) can occur only if the shell is between greater than about 15 kilometers thick. Measurements of the height of domes on Europa (up to a kilometer or just over half a mile high) also suggest that the ice shell must be fairly thick for the domes to be so tall.
However, it seems like there has been a some disagreement on this matter:
Some scientists have argued that the ice shell might be thinner, only a few kilometers thick.
But, the text doesn't seem to elaborate on any exact figures, so I'll look for some more sources.
Here's a paper that looks at Europa's ice shell:
Constraining the thickness of Europa’s water–ice shell: Insights from tidal dissipation and conductive cooling
From the abstract:
...when considering the present thickness of Europa’s ice shell, if tidal heating is included at a global dissipation rate of ∼1 TW, the shell is found to be, on average, approximately 28 km thick. However, if this dissipative heating is solely restricted to the shell, the local rate of heating may vary significantly due to crustal compositional heterogeneities and it is shown that this process may, in turn, produce thermal maxima in the crust, which could lead to local melting and structural instabilities...
So, with the combined information from these sources, I would stand by a figure of somewhere between 15-28 km. The reason I don't go any higher is because I don't see an error bound or range of values in the paper's abstract - it just says "approximately 28 km thick" - and the full paper is unfortunately behind a paywall. As the paper's abstract says, there are local heating phenomena that could occur and cause the crust to vary in different places, which could explain why the figure isn't that precise. The reason I don't go any lower, on the other hand, is because that NASA FAQ didn't specify any exact values for their mention of "few kilometers thick", and didn't provide much explanation as to why those said scientists supported that idea.