You are inaccurate in onething.

You take a number when your 485 is approved instead when you submit 485. The number is immigrately available to you when the cut-off date moves passing your PD. However, the number is not yours until CIS starts to request it immediately before you are approved. The actual need for the number is depended on how many cases are approved instead of how many 485s are pending. However, DOS uses the PD of the pending 485/140/LC to estimate the need of number, therefore, predict the future's cut-off date .