my free trial ends today or tomorrow. I don't know how many Users to pay for.

[Angela Gougeon JPREP]

I have one full time employee.  Then we use contractors for projects..

Contractors work for about 8 weeks at a time.   Can they be "guests" and access the Trello board for short periods?

[Mike - Dreamsuite Consulting]

You pay for workspace members and multi-board guests.  If you want Standard or Premium features by having a minimium of 1 person as a workspace member.

If the contractors only access 1 board, they can be single-board guests and not charged.  

If you require them to be on more than 1 Trello board, they will be multi-board guests and you will be charged. 

https://support.atlassian.com/trello/docs/board-guests/