Since you didn't ask specifically about the USA I looked for some local statistics instead. Solar-caused fires made up about 1.5% of home fires in 2016. That research was done because Australian standards for DC isolators were foolishly written and a number of fires resulted. So I would take the 1.5% (of home fires) as an upper bound.
https://renew.org.au/research/rooftop-dc-isolators-and-solar-system-fires/
https://www.pv-magazine.com/2021/07/02/australias-unsafe-solar-installation-standards-under-fire/
For industrial applications I expect the frequency is much, much lower because they use professional installers, do preventive maintenance, and any fires are likely to be less serious because they have staff on site at all times. It's similar to the difference in fire frequency between home generators and thermal power stations... not only are there fewer of the latter, fires are much less likely for a whole lot of reasons.