I think you're trying to do something like this?
RewriteEngine On
# Don't know if you need this, exclude www hosts
RewriteCond %{HTTP_HOST} !^www [NC]
# Make sure we don't already have a "cId" in the query string
RewriteCond %{QUERY_STRING} !cId=
# match the subdomain
RewriteCond %{HTTP_HOST} ^([^.]+).myapp.com$ [NC]
# add subdomain to URI as a query string
RewriteRule ^(.*)$ /app/index.php?cId=%1 [L,QSA]
This makes it so when you request anything starting with http://clientNo32.myApp.com/, it gets rewritten to /app/index.php?cId=clientNo32
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