Row count after SELECT INTO?

Dear SQL and plPgSQL gurus, I seek for your wisdom.

I have a variable assignment via SELECT INTO in my function, and I want
to separate 3 cases:
1) no rows found;
2) one row found;
3) many rows found (ambiguous select).

The first case is simple, I use FOUND variable for it.
But how about the third? I tried to GET DIAGNOSTICS nrows = ROW_COUNT,
bit it seems to get only value of 0 or 1. Is this how it supposed to be
when SELECTing INTO a single variable?

The only way I see now is a FOR ... IN SELECT loop, and I woner if
there is a simpler solution. Could you please help me?

--
Fduch M. Pravking