I'm not quite sure I understand what your question is... If you want to come out of your audio interface and have each output from that device routed individually to an external summing mixer, then you'll need to have a corresponding number of inputs available on the summing device.
If the Neve has more inputs than your I/O has outputs, there's nothing wrong with that, you'd just have extra inputs available for future use - which certainly isn't a bad option to have.
But you'll only ever be able to buss out the number of tracks or stems that your I/O has the outputs for.
Start with what you have and see what you think...even if you did have the ability to send 16 discreet outs to 16 ins of the summing mixer, if your DAW project has 24 tracks ( or higher), you'll still need to assign multiple tracks to some of the same outputs - this is where "stems" come in, for example, using 2 outputs of your I/O for a stereo mix of your drums, or 4 guitars, or 6 vocal tracks, etc.

Edit: I wasn't familiar with the Neve you mentioned, so I looked it up on Vintage King, that's a very nice summing mixer! Lots of cool options... Are you going to get the optional digital I/O connection with it? That would be a great feature to have and would allow you to mix to a second DAW for your final 2-Mix capture. There are several fans of this particular workflow (2 DAW Mixing) here on RO. ;)
-d.

Hey thanks so much, yeah I plan to stem out instruments passing through each time till im down to a stereo mix. Not sure what I'll get a thus stage I definitely need to do more research I neve heard of a two daw mix down Im definitely stopping everything im currently doing and looking that up. Thank for the help.

Arti, post: 455873, member: 51161 wrote: Hey thanks so much, yeah I plan to stem out instruments passing through each time till im down to a stereo mix. Not sure what I'll get a thus stage I definitely need to do more research I neve heard of a two daw mix down Im definitely stopping everything im currently doing and looking that up. Thank for the help.

You should start your research right here on RO! Look for posts by veteran members Audiokid and Boswell ( audiokid and Boswell )....
Do a search on the 2 DAW mixdown workflow, with discussion by these guys, who both have real time experience mixing this way.... there is a boat load of great info about it! ;)

The Neve 8816 fitted with the optional digital output module would make a good summing mixer + ADC input for the second DAW. It's important in this workflow that the digital output is captured by a second computer and not by the computer that is feeding the stems into the external mixer. This is because, to get the maximum benefits of the 2-box mix method, there needs to be no link between the sampling rates in use by the source and capture computers, even if they are nominally the same rate.

To get this to work correctly you would need an AES/EBU serial input interface on the capture computer. Many of the external S/PDIF interfaces available for PCs can either be configured for AES/EBU signal levels. Alternatively, you can make a simple attenuating adaptor that would work with almost any S/PDIF input device, including those fitted as standard on many PC motherboards.

Holly molly, thank you for this may just be a perfect mix flow for me to capture possible into my laptop.

This was so helpful thank you I'm searching up more on this on here thank you