Update: after reading your question again, I’m now thinking – maybe you meant “… buy 1 of any variation…” (i.e. no matter which variation)? If that’s the case, then you don’t need to enable “Use variations” checkbox from my previous post. I.e. in this case steps are:
1. Enable “Per product” checkbox in “WooCommerce > Settings > Maximum Products per User > Limits”.
2. Go to your variable subscription product admin edit page, and set limit to 1.
Please let me know if I misunderstood you.