two groups of 10 male volunteers

The subjects were two groups of 10 male volunteers sealed into a mock spaceship for two simulated flights to Mars.

（source：https://www.eurekalert.org/pub_releases/2017-04/mdcf-mcf041117.php ）

I have a quesiton here. Does "two groups of 10 male volunteers" mean:
(i) There were two groups of subjects, with 10 male volunteers in each group; or
(ii) There were two groups of subjects, with 10 male volunteers in total.

I have read the article, but it does not mention this infomation in other parts.

Thank you!

As written, it would normally be interpreted as (i), with 10 male volunteers in each groups. In general, when we have N groups of X, where X is some description, the description X would be for each of the groups.

Since some readers may not interpret things in this way, in your own writing, you could write something like "two groups of 10 male volunteers each" to make it even clearer what is meant.