Do the number of Sites in a Multi-Site have a practical limit?
I'd highly discourage this.
We've seen large Craft sites in the half a million to million elements (users, entries, assets, tags, categories, etc.) range. But even those maxed out at 20-30 sites.
Craft's multi-site was designed for sites that share a common information architecture and I'd say even 20-30 sites are pushing what the UI was designed for in the control panel.