Its a hard one to gage!
As Manchester Citys team is harder to get into then, say Southampton's.
Also certain managers of first teams tend to not trust young players much......(Pulis, Allardyce) whereas other managers try and build around youth (Pochetino, Martinez)
Also some teams have bigger squads then others, if an injury comes in then sometimes a kid takes his place (Southampton used to do this with Taggart, Chambers, Ward Prowse, Reid) a little less now as they have bought a bigger squad.
I think I read somewere that Manchester United have the most players playing in all 4 professional leagues in the country from there academy but very few make it past 5-10 league games for Utd and are moved on