There's no hard limit on the number of WordPress installs you can have on an account.
Note that you may be limited by the number of MySQL databases on the account, or the number of domains.
We would strongly suggest only installing one Wordpress per hosted website. The reason for this is to protect against malware attacks. Because Wordpress is open source, there are a few vulnerabiities in plugins, and there can be in the software if it is not updated.
Hosted websites on our platform are separated so that if one Wordpress site was exploited on one hosted website, there would be no access to any other hosted websites on your (or anyone else's) account.
If you want to run a second Wordpress site in a subdomain, you can add the subdomain as a separate hosted website by clicking 'Add a New Website'