How many nodes are enough for 24 terabytes of data?

See how efficient Vertica is with disk I/O Or you aren't executing a whole lot of queries or loading a bunch of data.

The general recommedation for Enterprise mode for 40 TB–1 PB is 4–100 nodes.

See:
https://www.vertica.com/kb/Recommendations-for-Sizing-Vertica-Nodes-and-Clusters/Content/Hardware/Recommendations-for-Sizing-Vertica-Nodes-and-Clusters.htm

How much much space are you currenlty using per node?

SELECT node_name, SUM(used_bytes) / 1024^4 used_tbytes FROM projection_storage GROUP BY node_name ORDER BY node_name;

Fyi... Here is the query MC uses to help generate the node threshold I/O alerts:

SELECT t.x, round(case when t.perc>=100 then 100 else t.perc end, 2.0) as y, node_name
  FROM (SELECT
                        timestamp_trunc(start_time,'MI') as x,
                        node_name,
                        max((total_ios_mills_end_value-total_ios_mills_start_value)/((extract('epoch' from end_time)-extract('epoch' from start_time))*1000))*100 as perc
                FROM v_internal.dc_io_info_by_minute
                WHERE start_time >= sysdate() - interval '5 minute'
                GROUP BY node_name, timestamp_trunc(start_time, 'MI')) as t
  ORDER BY t.x;

There is no limits how much data single node can handle.
Direct answer to your question - one node would do
Most likely, one node will not suit you, but you are not asking for anything else.