The same way you deal with every other repetitive task which scales linearly with number of customers: some combination of "automate, outsource, and eliminate".
I get less than 10% of the number of emails per sale that I did two years ago, which lets me keep up with the business despite it expanding over the years. This is mainly because I tracked what was causing the support emails and started to provide solutions which did not require marginal effort from me. Exactly what approach is appropriate depends on the particular problem.
Take my personal bugbear, license key delivery. It sucks but you have to do it, and huge numbers of customers will fail at it because either they don't understand the concept or their email provider redirects your license email to /dev/null. So you put it on the confirmation page. Solves the problem for some people, but not all. So you put a self-help license key lookup on your website. Solves the problem for some people, but not all. So you adjust your app's purchasing logic to include a random number as a parameter to the purchasing page, which you associate with the license key, and then you have the app poll the server for recently completed sales matching that random number when its license key entry screen is open. etc, etc
Other problems could be addressed by program patches, changes in written documentation, changes in business processes, etc etc.
If you consistently improve on that, support scales pretty impressively. (I support in the neighborhood of a hundred thousand trials a year and about 1700 customers, answering all queries by myself.)
I get less than 10% of the number of emails per sale that I did two years ago, which lets me keep up with the business despite it expanding over the years. This is mainly because I tracked what was causing the support emails and started to provide solutions which did not require marginal effort from me. Exactly what approach is appropriate depends on the particular problem.
Take my personal bugbear, license key delivery. It sucks but you have to do it, and huge numbers of customers will fail at it because either they don't understand the concept or their email provider redirects your license email to /dev/null. So you put it on the confirmation page. Solves the problem for some people, but not all. So you put a self-help license key lookup on your website. Solves the problem for some people, but not all. So you adjust your app's purchasing logic to include a random number as a parameter to the purchasing page, which you associate with the license key, and then you have the app poll the server for recently completed sales matching that random number when its license key entry screen is open. etc, etc
Other problems could be addressed by program patches, changes in written documentation, changes in business processes, etc etc.
If you consistently improve on that, support scales pretty impressively. (I support in the neighborhood of a hundred thousand trials a year and about 1700 customers, answering all queries by myself.)