How do I add a pay-as-you-go budget to the Developer plan to activate more cron monitors?

It is currently not possible to have more than one cron monitor on the free Developer plan.

Additional cron monitors require a pay-as-you-go budget, which is only available on paid plans. In order to increase your cron monitors (or any event category), you will need to subscribe to a paid plan.

More information on cron monitor billing is available on this page.