dlb Does this occupy my VPN slot?
Yes it does.
You can use something like NetGuard and also use an Adblocking DNS server at the same time. The DNS blocks ads for you, while NetGuard works like a firewall and allows you to block even more connections per app.
However you wont be able to add a VPN if you use something like NetGuard because it occupies the VPN slot.
If you need to have a VPN and the VPN's adblocking is not enough for you, then I think your only other option is to have the VPN and use an external DNS blocker such as NextDNS. NextDNS will block custom addresses but can't do it per app.