See here: https://ods.od.nih.gov/factsheets/Folate-HealthProfessional/, section titled “Health Risks from Excessive Folate”. There is an upper limit for folate from supplements or fortified foods, but no upper limit for folate from food because high intakes of folate from food sources have not been reported to cause adverse effects.
I’ve heard that too much can throw other mitochondrial functions out of whack. I heard that in a podcast by a nutritionist. While supplementing with methylated folate is good, you may need to balance it out with other supplements. I think they mentioned L-carnitine but don’t quote me on that. And any B vitamin supplements are best absorbed when taken together, so take a B complex as well.
Here’s a summary of how L-carnitine also helps with mitochondrial function.