According to a 2019 study in the Journal of Clinical Nutrition, 350 mcg per week or 50 mcg per day is enough to restore any mild Vitamin B12 deficiency. A 12-week randomized, double-blind, controlled trial on 40 subjects found no difference when taking a higher dose of 2000 mcg orally once a week over a lower 350 mcg dose.
Vitamin B12 is essential for proper red blood cell formation, oxygen transport, neurological function, and DNA synthesis. Since B12 is water-soluble, higher doses haven't been found to have any adverse effects with the excess naturally excreted from the body.
Although naturally found in fish, meat, poultry, eggs, and milk products, Vitamin B12 is generally not present in plant foods. National Institute of Health has a dedicated fact sheet on Vitamin B12, which I found to be dense, but very useful.