I want to create a 300 multi-site wordpress network using subdomains

300 sites within a multisite shouldn’t be a problem by itself. Just be aware that things scale out quickly when you are operating that many sites. In particular, the number of posts will grow fast, and depending on the type of site, the traffic can grow fast.

300 empty sites with no traffic can be run on any shared hosting, but as the post count and traffic grows, you’ll want to look into more robust hosting options.