You need `\set` (not `\pset`!) to set ***psql*** variables. Unlike assignment in a Unix shell the psql assignment is not capable of arithmetic operations. You *could* use the psql command `\!` to execute shell commands, but I'll suggest two different approaches:

###Option 1: Let Postgres calculate and set new variables with `\gset`

[The manual about `\gset`:][1]

> Sends the current query input buffer to the server and stores the query's output into psql variables 

Then you can interpolate the calculated numbers as text and everything works.  
psql code:

    test=# \set servernum 5
    test=# \set stride 300
    test=# SELECT :servernum * :stride AS minvalue, (:servernum + 1) * :stride - 1 AS maxvalue \gset
    test=# CREATE SEQUENCE key_seq MINVALUE :minvalue MAXVALUE :maxvalue;
    CREATE SEQUENCE

A nice blog explaining `\gset`:

- http://marxsoftware.blogspot.co.at/2015/08/postgresql-psql-variable-from-query.html

I calculated and set `:minvalue` and `:maxvalue` in a *single* `SELECT` to optimize performance. Still, the ***downside*** of this approach is that you need an extra round trip to the Postgres server.

###Option 2: Function with dynamic SQL for repeated use

You *could* use a `DO` statement, but ([the manual again][2]):

> Variable interpolation will not be performed within quoted SQL literals and identifiers. 

This would complicate string concatenation. I suggest to create a function and use `format()` for clean code. If it's for the current session only, you can make it a temporary function by schema-qualifying with `pg_temp.` So the function is only visible to your current session and dropped at the end of the session. Temporary functions are an *undocumented* feature - even though suggested by Tom Lane. Details:

- [How to create temporary function in Postgres][3]

So:

    test=# CREATE FUNCTION pg_temp.f_my_seq(_seq text, _servernum int, _stride int)
      RETURNS void AS
    $func$
    BEGIN
       EXECUTE format('CREATE SEQUENCE %I MINVALUE %s MAXVALUE %s'
                     , _seq, _servernum * _stride, (_servernum + 1) * _stride - 1);
    END
    $func$ LANGUAGE plpgsql;
    
    test=# \set servernum 5
    test=# \set stride 300
    
    test=# SELECT pg_temp.f_my_seq('key_seq', :servernum, :stride);

We *need* dynamic SQL wit `EXECUTE`. The manual about [***Variable Substitution***][4]:

> Variable substitution currently works only in `SELECT`, `INSERT`, `UPDATE`,
> and `DELETE` commands, because the main SQL engine allows query
> parameters only in these commands. To use a non-constant name or value
> in other statement types (generically called utility statements), you
> must construct the utility statement as a string and `EXECUTE` it.

Related answer on SO:

- [Error when setting n_distinct using a plpgsql variable][5]


  [1]: https://www.postgresql.org/docs/current/static/app-psql.html
  [2]: https://www.postgresql.org/docs/current/static/app-psql.html#APP-PSQL-INTERPOLATION
  [3]: http://stackoverflow.com/questions/4990622/how-to-create-temporary-function-in-postgres
  [4]: http://www.postgresql.org/docs/current/interactive/plpgsql-implementation.html#PLPGSQL-VAR-SUBST
  [5]: http://stackoverflow.com/questions/36025308/error-when-setting-n-distinct-using-a-plpgsql-variable/36025963#36025963