I currently run 30 sites similar to what you describe as small business sites with generally low, non-logged in traffic on a 4 gig vultr server for $44 per month.
Nearly all of those sites use cloud flare for DNS, so they receive the benefit of approximately 30 to 50% fewer requests in general. I also use w3 total cache, configured as recommended by cloudways.
Based on my system resource graphs, I would feel comfortable hosting as many as 50 or 60 total sites of similar usage.
If your client is going to transfer you all the sites, I would get two 4 gig servers and split your sites between then 60/40ish, putting all your e-commerce sites on the less-populated server.
If I had it to do again, I would go with digital ocean over vultr, simply because they provide more bandwidth and are $2 cheaper per month – that’s close to 5% savings…
Total cost for 2 servers < $85/mo. Total income: at least $125/mo. I’m not sure of your market, but I’d shoot for $10-$15/mo/site as a wholesale rate. Remember, this isn’t GoDaddy shared hosting at $4/mo. The stack provided by Cloudways is on par with the plans offered by WP Engine and I refer my clients to their pricing page when discussing hosting costs.
I also recommend using updraft plus to schedule off-site backups to Amazon S3 or similar. The cost is close to zero and it offers a lot of flexibility on top of the daily backups available through the cloudways panel.
You can bulk manage (including plug in, theme, and core updates) for a network of WordPress sites using a tool called Main WP which has a free plugin that works great and some paid add-ons that make your life easier.
You can use main WP also to install and configure updraft for offsite backups across your entire network.
These services of core and plug-in maintenance as well as off-site backups are additional services you could offer at your hosting client. The services are fundamentally no cost to you, but if you could tack on 25 to 30% extra for the value add, that’s a great opportunity.