Try 2 or 3 at tops. Rejecting at youth level is a lot easier than when massive six-figure wages are being thrust under your nose as well as the chance to play in the first team with far better players who can facilitate your own game. Real will be knocking soon enough and that's "every kids dream" where he'd be surrounded by stars.Mainoo has rejected City's approach several times over the last few years whilst playing youth level football if I'm not mistaken. I don't think we need to worry about him leaving, unless we still haven't progressed from our current level in ~5 years' time.
I'm more worried about whether we can find an elite level CDM for him and Bruno, before the latter starts declining eventually.
Removing City; it's still the logical progression for any player, which is why we need to get up to speed and not have him thinking along those lines further into his development.