When Reynold number is very high can we assume it is inviscid flow?

Reynolds Number is the ratio of inertial forces to viscous forces in a flow. Witout viscosity Reynolds Number has no meaning. at all.

For all viscous flows, Reynolds numbers generally define the domain of such flows (In pipes, channels, etc.) in which turbulence occurs. In most cases there is an uper limit in the Reynolds number of the turbulent domain.

Perhaps that is what you were thinking.

https://en.wikipedia.org/wiki/Inviscid_flow

Why did you ask the question?

The numerical value of the Reynolds number depends a lot on the characteristic linear dimension of the flow system and the velocity gradients that occur in it. In many systems (pipe flow is a good example) viscous turbulence, which completely alters the character of the flow patter and resistence, occurs at Reynolds Numbers far greater than one.

The flow around the aircraft wing can be adequately modelled as an assumed inviscid flow, but the flow in close proximity to the wing's surface (the so called boundary layer) cannot. There, in the extremely high velocity gradients that prevail, viscous forces dominate. Significantly these viscous forces determine the point along the wing surface at whick the flow separates from the surface greatly adding to the profile or form drag, and in extreme cases leading to a wing stall.

The Point here is that the main application of the Reynolds numner is the identification of conditions in the flow of a viscous fluid in which turbulence will likely occur, and not whether viscous forces nust be considered as you incorrectly suggest.