I take 20mg methylphenidate for ADHD daily and I am having allergies from the weather. Is benedryl safe to take and how much max dose?

Safe but. It is safe to take benedryl but be aware that the potential sedation of antihistamines may offset the stimulant effects of methylphenidate.
Yes. Don't exceed recommended dosage in product insert, usually 25-50 mg every 6-8 hours, be careful with driving/operating machinery, and exercise caution with multiple drugs.