The amp hour unit is a bloody stupid one for most things like this. Better to look at it in terms of watthours.
Taking an iPhone 6 plus as a reference, it has a 11.1wh battery. Garmin and Wahoo at really bad at giving actual sizes of their batteries (at least based on my quick Google), so let's roll a d6, and say 5wh for a full charge of a Garmin/wahoo.
PBP is essentially 4 days, but you'll start with everything charged, so effectively 3 days. So if you need to charge each device once, that's (11+5)*3 = 48wh. Assuming 100% efficiency. That would be roughly equivalent to a 14000mah battery. But it's not 100% efficient, so maybe go for something closer to 1800mah.
Do do the maths yourself, based on better numbers, but gives you an idea.
I have an Anker with 98Wh of capacity with me. Because I'm charging a lot of devices (wahoo, phone, light, tracker, camera, other light). I'm thinking my 60wh per day power budget may have been a bit pessimistic. Some would say having a 500g battery pack contributed to my failure on the TCR. I'm inclined to think that in the temp, a bike with naught but lights would still have been walked up the hills.
Work out your power budget, and see what you come up with.
J