What do you mean by "shared accounts" ? Every account is individual - per portal/domain, regardless if it's free subdomain of FQDN. Per portal design can use php design, FTP upload or online editors for custom php edit, if portal is indeed made in php, but that does not apply to all scripts, just if you prefer your portal to have php as design concept.
There is no limit in this, but just what your browser looks for in public_html when opened...
Brod@Croatia