You say
I'm trying to compare how many rows there are in the table before the trigger and after the trigger

You don't actually need to query the whole table, which is very inefficient. You can just compare inserted and deleted

DECLARE @diff bigint =
    (SELECT COUNT_BIG(*) FROM inserted)
  - (SELECT COUNT_BIG(*) FROM deleted);

This works in a trigger which handles all of INSERT, UPDATE, DELETE .

If you have just a INSERT or UPDATE trigger you only need to query inserted .

DECLARE @diff bigint = (SELECT COUNT_BIG(*) FROM inserted);

If you have just a DELETE trigger you only need to query deleted .

DECLARE @diff bigint = (SELECT COUNT_BIG(*) FROM deleted);

If you need the full amouont of rows in the table, the fastest (if not always perfectly accurate) way of doing it is:

DECLARE @TotalRows bigint = (
    SELECT SUM(st.row_count)
    FROM sys.dm_db_partition_stats st
    JOIN sys.tables t ON t.object_id = st.object_id
    JOIN sys.schemas s ON s.schame_id = t.schema_id
    WHERE t.name = 'MyBigtable'
      AND s.name = 'dbo'
      AND st.index_id < 2
);

This result is after changes have been applied, so subtract the previous result to get the before value.