It depends on what level I am at in the game - I am currently League 1 England, small club (Borehamwood) and am building a roster that looks something like:
1st Team = 2x plyrs for each position -selected on CA alone.
PLUS 1 for each position who are under 23, train with the firsts and play U23 or out on loan.
U23s = At least one in each position, plus those who train with the 1sts, plus promising U18s
u18s= The most promising are moved to the U23 squad but available for u18s games. All intake are retained at least two seasons
Coaches are the best I can get to work for me
Seems to work at L1 level with good cover for first team, but I don't look for 'success' with my age groups.
If I was big Premier club I would be more structured about the youth progression and look to buy in for U18/U23 as well.
Lower league I would be much smaller roster and only consider bringing players in that directly improve my 1st 11.