At what rate do chromosomal rearrangements occur?
Obviously, there won't be a single number for this.
The phenomenon is fairly well understood in yeast. Unfortunately, most studies tend to focus on the rate of formation of specific well-understood rearrangements, and then use that as a baseline. I am having trouble finding a measurement of overall rearrangement rate, although the system is ideal for this.
There has been some work on characterizing rates of rearrangement (and its evolution) across evolutionary history, I don't know if that is directly relevant.
This paper has a decent breakdown of the incidences in humans across different kinds of translocations. For example:

Additional studies (5, 6) followed and the incidence of balanced translocations was ultimately determined to be 1 in 500 in the general population.


Studies using cleavage stage biopsy for reciprocal translocation PGD have reported proportions of unbalanced embryos as high as 82% (57).

There is good reason to expect that it varies significantly across species and across chromosomes. For example, the Drosophila Y is a hotspot.
And (first paper):

Rearrangements involving chromosomes 13 and 14 (der(13;14)(q10;q10)) account for 75% of Robertsonian translocations (12).

For an early paper looking at a large population, you can see here.
Note that most of these human numbers are indicative of population incidence rather than rate of occurrence. Others have attempted to study effects on the translocation rate directly.
The best citation that I found for directly measuring translocation rate is here. However, the data are apparently for somatic translocations and are rather unevenly sampled so I am unsure of how generalizable it is. I looked at WYSIWYG's papers and they might provide some specific examples but they don't seem to address translocation rate directly either.
Take-home, it's hard to directly measure rearrangement rate in humans due to inviability of many/most aneuploidies early after fertilization. Somatic is easier to estimate but may not be representative of rearrangements in the gametes.