Does that mean there are 428 domains hosted on the same server as ours?
Don't confuse web hosting and DNS hosting. They are independent services and usually hosted by separate servers. They have separate administration and quite different resource requirements.
When the 'whois.sc' website talks about nameservers, it strictly means those which host your DNS zone (basically: IP address mappings for the domain and subdomains). It is common for this DNS data to be hosted on a shared server owned by the domain registrar (possibly even holding tens of thousands of zones), and many registrars provide this as a free service.
However, that's completely independent from where the domain's own services are kept. It doesn't tell you anything about whether the website runs on a beefy dedicated server or on a potato.
Instead, to get a guess whether the web server for your main domain is shared or dedicated, you can look at the "IP address:
" line slightly below in the same whois.sc result page. If whois.sc has seen other domains pointing to the same address, it will say "X other sites hosted on this server". If it hasn't seen any yet, it'll say "is hosted on a dedicated server".