It should be possible to count the number of different ones fullVisitorIdgrouped by month:
SELECT
EXTRACT(MONTH FROM
TIMESTAMP_MICROS(user_dim.first_open_timestamp_micros)) AS month,
COUNT(DISTINCT user_dim.app_info.app_instance_id) AS monthly_visitors
FROM `your_dataset.your_table`
GROUP BY month;
( , ). + :
SELECT
FORMAT_TIMESTAMP(
'%Y-%m',
TIMESTAMP_MICROS(user_dim.first_open_timestamp_micros)) AS year_and_month,
COUNT(DISTINCT user_dim.app_info.app_instance_id) AS monthly_visitors
FROM `your_dataset.ga_sessions`
GROUP BY year_and_month;