I need to increase the number of Cron monitors but I cannot do this above 1.

Every Sentry organization has access to a single monitor for free.

Any additional monitors will cost $0.78 USD/billing cycle, and are accrued to your on-demand/Pay-as-you-go spend when activated.

You can configure an On-Demand spend / pay-as-you-go at the bottom of your Subscription page if you are on a paid plan.
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If you will only be using On-Demand for Crons you may wish to configure per-category on demand.

Note: per-category on demand has been deprecated and is not available on all plans.