This depends on a lot of factors—Server resources, traffic spikes, the website itself, etc. It’s difficult to make even a conservative estimate. All best web hosting companies prefer to talk in terms of resources rather than actual visitors.
Only a few providers give a rough number. SiteGround claims its largest plan is suitable for about 100,000 visitors, while HostGator’s number is 300,000.
You can usually track resource usage from the dashboard. It’s best not to worry about this early on, though.
If a provider lets you know you need to upgrade your plan, it’s actually a good sign. It means your website is growing.