It's generally safe to take up to 2 doses of antihistamines per day and anymore than 2 antihistamines per day could cause an overdose.
As for the milligrams for dosage of antihistamines you should take no more than 5 or 6 milligrams of antihistamines or it could lead to an overdose of the antihistamines.
For me I take usually 1 antihistamine per day unless I have more severe allergies and the one antihistamine dosage is not working.
Then in that case I sometimes need to take 2 antihistamines to help with the allergies but my doctor told me I should not take more than 2 antihistamines or it could lead to an overdose.
As long as you don't take more than 2 antihistamines per day you'll be safe and if you do take more than 2 antihistamines you should see a doctor or go to urgent care if you notice any complications.