How many SSDs/HDDs can I connect to the Antec Earthwatts 500EA PSU?

I want to add an internal Serial Ata drive to my server but I do not know whether I have enough power in the Power supply unit . I have an Antec Earthwatts 500EA PSU. It has 420 continues power and 500 watts maximum. How many internal sata disc drives can I connect to my system ? my config is as follows :
- 7 Western Digital enterprise internal sata drives (in RAID 5 config).
- 1 Samsung SSD 850 pro (booting)
- 1 Asus GTX 260 (old but still working)
- 1 DVD writer
- 1 Creative labs extreme music PCI card.
- Intel CPU : Skylake 6500 (using ± 50 watts)
- 8 GB DDR4 RAM.

is is safe to still add another internatal drive for Datacenter usage ? (have a server computer case)

Edit by Fernando: Thread title shortened

Based on your data I think you can, but again, it’s all about testing