I want to create a rectangular cluster of many circles and apply a color gradient to fill these circles.

When I select the group of circle objects, click the gradient tool, and create the gradient line, Inkscape crashes when if have 19 or more circles selected (but not fewer).

For example, in the image attached I can apply a gradient to the top group but the bottom crashes:

I'd ensure sharing of gradients is enabled.

I'd also group the objects before applying the gradient.

(After ungrouping.)

Actually, I updated in version to 1.2.1 from 0.9.something, and it seems to survive the original approach now.

What do you mean by grouping first though? When I select them all and click Object > Group, then try to apply a gradient, it behaves oddly and turns all objects black.

Before proceeding further, I'd reset the preferences.

I group them to apply the gradient spreading from objects on one side to objects on the other, rather than the gradient spreading across each object individually.

The default gradient on groups is black because groups have no fill or stroke, but setting the gradient on that group will apply it to all objects contained in the group.