I'm not sure if you understood my comment fully since none of the benefits I gave have to do with number of devices. Again, the main reasons I listed are that dedicated router boxes on x86 hardware is much more flexible configuration-wise and has many more software packages and addons that can be easily installed compared to consumer routers. You can have plugins like nginx, radius, wireguard, snort (as well as full power to run ids/ips at full speed), etc. For configuration, you have more control over multicast, ways to customize local dns resolution, the full range of local hostname resolution settings, DNS failover, multi wan with the full ability to tune failover metrics, advanced routing rules using hostname aliases that periodically auto-update, advanced dhcp flags and dhcp6/SLAAC settings, virtual IPs (a huge help when doing 0-downtime migrations between hardware or subnets), network bridges, GRE and LAGG, v6 router advertisements, and so so much more.

If I had a consumer combo router there's a good chance I would not have vlans, all my roommates would see each other's smart devices and it would be pretty annoying. I wouldn't be able to selectively route only traffic to google servers from only my laptop, phone, and chromecast through the same Germany VPN so that all the non-google traffic would be unVPNed, and I wouldn't be able to set multiple multi-wan failover modes (let alone gateway groups to group failover WANs) so that for example one vlan fails over from the fiber connection to the copper connection while our neighbors connection fails over from the copper connection to the cable internet connection. I would have no ingress load balancer on my router handling incoming traffic to my homelab, and I would have to use extra media converters to get my SFP+ fiber connection to connect to a consumer router's 2.5G port (did we even have consumer routers with mgig 4 years ago? That's around when I got my fiber).

None of this has to do with number of devices, but total capacity is a bonus of having nicer hardware than consumer crap. This wouldn't be a benefit to most people, which is why my main points are about configurability and flexibility with third party packages, but it is a benefit to me since I have 4 gig of total wan and a 10G link to my core switch. If any 2 of the 10 people in this apartment decide to download from steam at the same time, they will both get a full gig download with plenty of bandwidth left over for the other 7 people to be streaming or doing whatever. Again nothing to do with number of devices, more to do with how many simultaneous high-bandwidth uses you expect to coincide. Of course I could just have everyone share a single gig connection (or 1.2 gig which is currently the maximum residential plan you can get here), but then I would need to deal with traffic shaping / queues, another thing that opnsense coincidentally excels at, having way more traffic shaping options. You can even do traffic shaping on a per-destination basis - for example you could use an auto updating ASN alias to categorize traffic to steam or netflix, then dynamically apply different traffic shaping rules based on which user is accessing those services.

TL;DR, consumer routers cannot come close to achieving a fraction of the configuration options that open router platforms have. While you might see benefits in capacity if you invest in a good uplink and high end APs (I have uap u6 pro which is "good for 350 devices", though really I bought for the higher single device performance and higher modulation rates and better mimo configuration), even people with slow internet and very few devices can benefit from the immense amount of configurability that these OSes provide - you're practically one step away from running a bare OS with open source packages installed and editing a slew of config files where you can use every obscure configuration option that any of these FOSS contributors ever put into these daemons. In fact many of the opnsense configuration pages have an advanced text box at the bottom where you can put in extra config directives in case the UI doesn't include a knob for something you need.

It's great, 10/10 recommend opnsense or pfsense