WordPress is a popular website builder, but it's not a website host, so you can't limit the number of users a website can have. However, there are some essential steps that must be taken to ensure that a website does not crash when too many people try to visit it. WordPress itself has no limit to the number of visitors at the same time they can visit a WordPress website. You mean a private blog? You can add 35 users to your private blog and you can purchase the Unlimited Private Users upgrade if you want to add more.
If you are referring to a public blog, there is no limit to the number of official users you can add to your blog, adding users and user roles. Yes, WordPress can handle a lot of traffic. WordPress itself has no limits on the amount of traffic it can support. However, your choice of web hosting and on-site performance optimizations can make or break your site's ability to handle heavy traffic.
It's not that WordPress itself is slow, but that many site owners use slow themes that burden their sites with unnecessary features and make them fat. GoGeek is a “semi-dedicated plan” that includes SiteGround's WordPress SuperCacher, free site migrations, on-demand backups, staging, high priority support, PCI compliant servers, free SSL and Git. Not to mention that with so many users you probably already have a large number of posts, unless your application is something that mainly deals with users (CRM, etc.), and you choose to store your user data in wp_usermeta instead of wp_postmeta. In fact, some of the biggest websites on the Internet today with millions and millions of users are powered by WordPress. It seems that the number of users outside of spammers is just a reflection of how active (read popular) your website is. Above I have given you quite a few ideas and tips to improve the ability of your WordPress website to handle heavy traffic and millions of users, if you are so lucky to get them.
Another great way to optimize your WordPress site for heavy traffic is to use a content delivery network, also known as a CDN. Assuming that all users have at least 1 role, they have a wp_capabilities entry in the user_metadata table with a serialized array of roles. New WordPress updates and different plugins sometimes include more features, some of which can help make your website more efficient. How much traffic any WordPress website can handle doesn't depend on WordPress, the CMS itself, but on where it's hosted. It is not the content management system that determines how many users your WordPress website or any WordPress website can handle, but rather it is the resources of the server and your web hosting provider that are likely to determine how well and how many users your WordPress website can handle. If you need to host any non-WordPress application with substantial traffic, you'll want to look for dedicated servers or cloud hosting. I have found that the bottleneck for how many Wordpress users you can have is the PHP timeout that comes into play on the user administration page.