My companies have supported X00,000 users and ~8k paying customers over the years with a maximum team size over that interval of, ohh, 1.25 or so. It's quite achievable by a small team.

I did all my own CS, over email, for the first 6 years. This was approximately 4 years too long. Time to first response (TTFR) was generally on the order of "I'll get back to you by the next business day on a best effort basis" -- I hit that on about 98% of customers for the first few years and then probably slid later, particularly when sick or overwhelmed.

My first (B2C) product had a lot of not-very-critical CS issues which also weren't very complicated to resolve. After having done that for 6 years, I hired a VA firm in the Philippines who staffed me with a single person for ~4 years. I think it was 20 hours a month for $300 or so, IIRC. I acted as tier 2 support for her. She probably head off more than 90% of inquiries. We used Snappy (BeSnappy) for CS software -- I enjoy it rather more than I do support over email, and it was easy enough for her to get spun up on.

I've also done B2B support for my other SaaS product, which has fewer users, fewer customers, fewer incidents, and radically higher maximum-criticality-of-a-ticket. Again, did it all by myself for the first few years. Eventually it became obvious that a supermajority of our support was onboarding for new customers, so when I hired a sales manager we mutually decided to roll that into her job description. Our process is she firewalls me from any ticket she can and, if anything survives, she dumps it into Slack as a morning roundup every evening. I either get her instant answers or tell her "#3: ETA tomorrow."

My sales/support person works near my customers timezone-wise, so they generally get very quick "Thanks for the email. I'll ask my tech guy about that and get back to you." acknowledgement for the harder issues (and very fast resolutions to the easier ones). This makes for mostly happy customers, although to-be-fair most of them thought I was pretty responsive back when they wouldn't even get first contact until a day after they had sent in the ticket. I'm not competing with having a super-responsive startup team on speeddial; my customers expectations' are set by bureaucracies which take weeks to acknowledge receipt of the first of three things required to get the ball rolling on opening a ticket.

Fully-loaded cost for my sales/CS person is on the order of "a few thousand dollars a month." for what I'd estimate as approximately 0.25 FTEs. (Worth every penny in decreased stress level for me.)

Customers occasionally ask for 24/7 phone support. I quote Enterprise rates for anyone who wants that. No takers, thankfully.

I feel like the median experience of my customers getting CS from the team and myself is probably worse than it was when I was doing 100% of the CS, but it is not obvious to me that is true of their perceived experience, and we've managed to successfully manage expectations from customers about how much of my personal time is included for $29~$199 a month. (A particularly good thing as I won't be involved in that business much longer.)

Anecdata: 80% of CS incidents take < 5 minutes to resolve, 15% take < 20 minutes, and 5% take Way Too Long.

Favorite tricks for CS:

You're going to get the same issues a lot. Keep a list of them. Fix the ones that are caused by fixable product issues, and keep fixing them until those go away.

Build self-help tools for the issues which are high-frequency low-value-added like, most obviously, password resets. (Yes, there are still companies without automated password resets. I know someone with a master's degree who carries a beeper on Saturdays due to their company's refusal to spend money on tweaking an application to allow password resets.) Routine billing inquiries, etc are often also candidates for this.

You're now looking at a long tail of issues, but it still has a fat head of the distribution. Process-itize those issues which are amenable to it: you want to have a Google doc which explains exactly the company standard response to X, including what to say (templates optional) and what, if anything, needs to be done in internal tooling. Do not extemporize a solution to the same problem 100 times: cache it and save extemporization for places where the team's brainsweat actually adds value over "the right answer delivered very efficiently."

Final thoughts: your expectations for CS are not your customers' expectations. Your expectations for your company's CS are not your customers' expectations. Your desired outcome for a CS incident is not your customers' desired outcome. I can keep banging this drum for a while. It's important.