How do I add on-demand/pay-as-you-go budget to the Developer plan to activate more crons?

On-demand/Pay-as-you-go budget 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.

You can read more about the pricing for Crons here: https://sentry.zendesk.com/hc/en-us/articles/23058282687259-How-does-the-pricing-for-Crons-work