How many members can I invite? Is there a limit?

Each member has to have a subscription, so the limit is your budget.

我是我的我 said:
Do invitees also have to subscribe?

 Yes, anyone who you want access to your DBs will have to have a subscription to ninox. It can be at any level.

There is no way for someone to have direct access to your DBs without a subscription.

If you have a website, you can provide access through integration software like Make or Zapier that can provide access to Ninox through a website.