How many Prestashop platforms can be connected to Ergonode at once?
There is no limit.
Is the synchronization between PrestaShop/Baselinker platforms one-sided or two-sided? (Will new products added to Prestashop or Baselinker sync with each other?)
Synchronization is between Ergonode > PrestaShop and Ergonode > Baselinker. Not between connected e-commerces. Products added in one e-commerce will not add in Ergonode nor other e-commerces.
When syncing with PrestaShop, will variant and grouping products transfer to e-commerce?
Yes, but as simple products.
What happens when, having a product already synced with PrestaShop, we remove the photo assigned to that product in Ergonode?
When you sync again, the image will be deleted in the product in PrestaShop.
How can I avoid creating all categories form Ergonode in PrestaShop?
In Ergonode create categories tree and pin to it only those categories you would like to sync wiith Prestashop. Don't forget to map it in connector configuration.