I totally get that this is really hard to manage - that’s what makes it such an interesting challenge 
According to an a blog post from a few months ago, a single COp can manage about 3k users each. Which means that you’re hiring enough people to manage about 21k users per week. That would be about right, since Monzo’s growing by about 20k users per week at the moment.
But that’s assuming that:
- Efficiency hasn’t improved - hopefully the tooling & training’s got better since then
- You’re already able to increase response times now by routing conversations to specialists (presumably if you weren’t, that would be slowing you down at the moment)
- The switch from Intercom to Monzo Chat isn’t impacting response times
- The number of COps per user hadn’t already fallen behind (based on the current response times, it seems like it had)
- The new COps reach their full potential straight away - as you said, it’s not realistic to expect that they will
- You continue ramping up hiring as Monzo’s user growth per week continues to increase & ideally (perhaps) begin ‘overhiring’ in anticipation of that growth
And then the more interesting question for me is: whether this is having a significant impact on user’s satisfaction - it looks ‘bad’ to me that I have to wait 3 days to report a bug but since that’s not urgent, it doesn’t upset me & response times for urgent queries are still great 
TL;DR - I’m looking forward to a deeper dive into what’s happened here & how Monzo feels this has gone later on 