If you create a linked record and a roll-up field then you could use the solution in this post (https://community.airtable.com/t5/show-tell/count-unique-values-in-rollup/m-p/113668#M1834) potentially.
You can create a "Months" table, which will contain the list of months.
In the "Months" table, create a lookup field to the table where you keep track of volunteers. You can set up an automation to automatically link each time someone volunteers.
Then, you can simply create a Rollup field that will count the unique volunteers.
Example formula:
COUNTAll(ARRAYUNIQUE(values))
Result:

If you need more help feel free to schedule call with me.
Hi,
For a quick look, you can choose unique

and group by month with 'Collapse all'

But if you want to use these numbers per month, you should follow link provided by @Ella11
Hi,
For a quick look, you can choose unique

and group by month with 'Collapse all'

But if you want to use these numbers per month, you should follow link provided by @Ella11
Thank you! That is a simple way to get what I need - which works for me!