How to Limit Number of Simultaneous Queries by User in PostgreSQL

I try to answer your question - to be correct, I try to answer the question I think you've asked (see the comments). Until then I'll assume that a virtual table is simply a table and you want to count the tables a user has SELECT privilege on.

First, you have to collect all roles which the given user belongs to. This will include the user itself, all roles that are granted to it, all roles that are granted to the roles granted to your user etc., and finally, but usually most importantly, the PUBLIC role.

You can collect these from the information_schema.applicable_roles view with a query like this:

WITH RECURSIVE privs(grantee, role_name) AS (
    SELECT grantee, role_name 
    FROM information_schema.applicable_roles 
    WHERE grantee = 'my_user'

    UNION ALL

    SELECT ar.grantee, ar.role_name
    FROM 
        information_schema.applicable_roles ar 
        JOIN privs ON ar.grantee = privs.role_name
)
SELECT role_name FROM privs

UNION 

SELECT 'my_user'

UNION 

SELECT 'PUBLIC'

Then you have to consult the information_schema.table_privileges view. In order to see all the rows belonging to all grantees, you have to be logged in as a superuser. With an ordinary user you will see only the roles granted to your user (including indirectly granted roles, ie. if the grants are user -> user1 -> user2, you will see user -> user1 and user1 -> user2 if logged in as user, and only user1 -> user2 if logged in as user1.)

Here you can look for tables with SELECT privilege like

SELECT table_name
FROM information_schema.table_privileges
WHERE 
    privilege_type = 'SELECT'
    AND grantee IN (... the recursive query above goes here ...)
;

Turning this into a count is left as an exercise for the reader :)

When we configure the memory setting for the Postgres DB what is the recommended memory allocation for each connection ? Is there any formula to apply ?

There is no per-connection pre-allocated memory that a DBA would define. A session will allocate dynamically what is needed, or use the shared memory pre-allocated at server start depending on the kind of usage. See Resource consumption in the doc about what can be defined.

I know 25% of the memory of the Server should be allocated. But how do we allocate based on the DB connections? How do we know, what is the maximum number of connections should be allocated?

The maximum number of client connections (max_connections) is factored into the amount of shared memory that is pre-allocated at server start.
In Managing Kernel Resources, the doc says that its size in bytes is:
(1800 + 270 * max_locks_per_transaction) * max_connections

But memory is not the main factor in deciding the maximum number of connections, it's the raw power and number of cores of the machine, and wether a connection pooler is going to be associated to the server. The point is mostly to avoid too many concurrent active queries provoking the server to halt to a crawl.

Also in a multi-nodes environment can we allocate more connections for each node (in Postgres-ds.xml max connections) than what is actually allocated in the DB?

If there are multiple client nodes that point to the same PostgreSQL instance, the combined count of their connections cannot outreach max_connections.

However when using a connection pooler, connection as a word becomes ambiguous. You want to know whether it's a connection from the client to the pooler or a connection from the pooler to the database server. Generally there are much more allowed connections to the pool than to the database.