Your AdMob account has maximum limits on areas like the maximum number of active ad units you can have. While you'll likely never notice them, keep these limits in mind when planning monetization, campaigns, and mediation in AdMob.
The following table shows the limits for different areas of your account.
|Mediation groups||500 active or paused|
|Ad source instances per mediation group||50|
|Ad units per mediation group||250|
|Campaigns||10K active or paused|
|Campaign ad creatives||4M active or paused|