Is the vitamin b12 level of 194 pg/ml too low for 51-year-old female? should supplements be taken at this level?

194 is low -normal is more than 200 . I like 500 range. Would discuss with MD if should get injections initially and then move to oral supplementation. Evaluate why so low. Check methylmalonic acid level as well. May be related to diet ( vegan) , pernicious anemia or may be terminal ileum issue ( inflammatory bowel)f/u/w/ MD https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/

Yes. A level of 194 is lower than desired. You may try sub-lingual preparation of B12. If that does not resolve the issue in a month or so, you may need injectable B12.