Hi hardy,
By default, WHMCS generates renewals for services that are due on the same day into a single invoice.

You can use Signup Anniversary Pro-rata feature to prorate a client's services to all renew together: https://docs.whmcs.com/system/general-settings/general-settings-ordering/#signup-anniversary-prorata

For existing services, use the Bulk Actions on thr Client's Summary tab to prorate them to a single date: https://docs.whmcs.com/clients/client-profile/summary-tab/#bulk-actions