A SATA power splitter will be what you need. https://www.amazon.com/Cable-Matters-Pack-Power-Splitter/dp/B012BPLW08/

Try to find something like this that doesn't have the molded connector, those can heat up, short out and catch fire.

The motherboard has 8, and the HBA card adds 8 more.

I have an EVGA Supernova P2 850w with 4 Connectors.

And I use these splitters. Cable Matters 2-Pack 15 Pin SATA to 4 SATA Power Splitter Cable - 18 Inches https://www.amazon.com/dp/B012BPLW08/ref=cm_sw_r_apan_glt_fabc_Y3ZYQYDMHC66CJNR1AXW

What exactly do you mean by that? If you just want to connect more drives you can generally just get a SATA-to-many-SATA adapter and they will work just fine. You only need more watts on the PSU if you're at the limit for your system overall, the SATA line will draw as many amps as needed to run all the drives connected to it.

I’ve just been buying these and taking them apart to make my own

I have an EVGA Supernova P2 850w

And I use these splitters. Cable Matters 2-Pack 15 Pin SATA to 4 SATA Power Splitter Cable - 18 Inches https://www.amazon.com/dp/B012BPLW08/ref=cm_sw_r_apan_glt_fabc_Y3ZYQYDMHC66CJNR1AXW

Hmm, I use something like this on a Corsair 750D but the cables on mine have smaller spaces than the product page on Amazon so they're a better fit for drive caddies. (the one I used I got from a local computer store)

Could you use this which comes in a 2 pack: https://www.amazon.com/Cable-Matters-Pack-Power-Splitter/dp/B012BPLW08/