If you allow me to ask please, what is the size of your instance (user base, and storage)?

7 users per day and 20 per month. The hard drive is 40 GB currently.

what are the maximum concurrent users at the same time?

I remember here being some ~20 concurrent users.

can 6euro vps handle good amount of posts, users and activity?

Yes.

do you suffer problems during the spike?

No.