Return Number Not String In Dynamic SQL
You could do this with sp_executesql and assigning the variable to an output parameter

DECLARE @numreturned  INT;
DECLARE @sql NVARCHAR(MAX);
DECLARE @fulllocation NVARCHAR(MAX) = 'nagadishi.mirama.dbo.Humizan';

SET @sql = N'SELECT @numreturned = COUNT(*) FROM '+@fulllocation+' '
                   +N'WHERE ([Nishan] IS NULL)';

EXEC sys.sp_executesql @sql, N'@numreturned INT OUTPUT', @numreturned = @numreturned OUTPUT;
SELECT @numreturned;