How to increase the number of activity in pg_stat_activity?
pg_stat_activity has one row per server process.
See PostgreSQL documentation in table 28.1:

pg_stat_activity One row per server process, showing information related to the current activity of that process, such as state and current query.

and later down:

The pg_stat_activity view will have one row per server process, showing information related to the current activity of that process.

If you want more, you need to increase the number of connections (backends) which default at 100, see this other part of the documentation

max_connections (integer)
Determines the maximum number of concurrent connections to the database server. The default is typically 100 connections, but might be less if your kernel settings will not support it (as determined during initdb). This parameter can only be set at server start.