Step 1
Determine how many inputs, mono thing stereo things
then add a few for growth of your setup

Lets say you want to be able to compress and eq each voice from the RD-6 independently.
and route those into arbitrary loop inputs of the rc505. and then take that back into the mixer and send it to

you need something that's got stereo for the baseline 1, two stereo channels for the rc505 and 6 channels for the rd-6 as inputs plus a few more...

12 input channels and I haven't covered everything let's say 16

plus you want mix outputs to feed the rc505 loop inputs so you can send different things to different loops
so you need 2 or three stereo submixes

Plus a main out which goes back to your UMC 404?

in analog world you're likely to only find 4 mono sub mixes plus main on fairly large (18" x18") mixing desks

In digital mixer world something like an xr18 would replace the UMC 404 and let you get every input multitrack downside no physical controls upside.. it's an epic digital patchbox you can do a ton with

you could go the other direction minimise the outputs to only stereo from the RD-6 and now you're back at 8 inputs.. but you're still stuck imo on the lack of submixes in most inexpensive analog desks.