Should I be concerned about an mchc level of 38.5 from a CBC test? I know it can be a deficiency but I've been taking a multi with B12 for 4 months.

Not MCHC pre se. There are other causes of large red cells, in addition to B12 and folate (folic acid) deficiency, including some medications. It would help to know the whole cbc. All laboratory results need to be interpreted in the clinical context and the doctor who ordered the tests is usually in the best position to do that.
Ignore, U need to look at the CBC report before one may suspect vit B12 deficiency. To suspect macrocytic anemia the MCV need to be >100.