What vitamin deficiency mostly affect patients with crohn's disease and why? is it only the b12?

Not only B12, but the deficiency of other fat soluble vitamins, minerals and electrolytes may occur due to poor absorption and it depends on how severe your disease and what section of the small intestine is affected the most. Severe colonic disease will cause significant iron deficiency due to chronic blood loss. B12 deficiency occurs when terminal ileum is severely involved in Crohn's disease.