Very good question to discuss . There is no such limitation set by Microsoft as of now for maximum number of hub and associated sites association. However, the maximum number of hub sites in tenant is 2000 as of now, this limitation got increased over a period of time (earlier this limit was just 200).
Now, let's understand the architecture of hub sites why do we need it?
The simple answer is - Microsoft wants to get rid of from nested architecture (sub site based) which has been using over a decade. Microsoft recommends flat architecture, simple meaning is no more sub site, by default all sites are considered as individual site collection.
Then, how to manage n number of sites from single place?
As we know hub site is not a different site object, it is a normal SharePoint site where we can promote any existing site as a hub site. When we have multiple sites, then managing those sites would be a real challenge - for this Microsoft came up with Megamenu, so megamenu and hub site is integrated part of modern SharePoint online.
What is the real business needs for hub sites and associated sites?
Apart from flat architectural benefit, the real benefit is grouping the sites based on the business domain.
For example, let's say we have, 10 HR related sites, 10 custom sites where more customization is done, 10 document management related site and so on.
Then, we can group the hub and associated site as below :
HR Hub(hub site) - Here all 10 HR related sites will be associated to the HR hub site.
Custom Application Hub(hub site) :
Here all 10 customized businesses application related sites will be associated to the custom application hub site.
Document management hub site : Here, all 10 document management sites will be associated to the document management hub site where main core business focus to have various types of invoices related do.
For each hub sites and their associated should have a dedicated
site owner those will be responsible to maintain their respective sites - for this we can configure a site owner web part at hub site
and associated sites home page.
Following the above ways, we can better way can manage and get the
real business benefits of hub and associated sites.
To conclude this, there is no limitation in terms of numbers in hub and associated sites, so, group the sites based on the business areas and map the hub - association and continue to use as many you want. :-)
For details hub and megamenu refer to the below articles :
Why should we create a hub site in SharePoint Online (benefits of SharePoint hub site)?
How to create navigation in modern SharePoint Online