According to the information on the main website, drinking coffee or tea while fasting is fine. I think that each person would be the best judge of how much coffee that they should drink.
Myself, I usually have about one cup of coffee a day, two at the most; so if I started drinks a lot more coffee, I am sure that it would have me feeling wired up and shaky. Someone who is used to drinks multiple cups of coffee each day would be just fine drinking more cups of coffee.
I think that the main thing here is to be sure not to add cream and sugar to the tea or coffee, so that you do not spike insulin and also break your fast.
https://omaddiet.com/omad-diet-liquids/