So if there are 20 slots and 20 reserved slots but no one online, that means people who don't have a reserved slot can't join?
No if the server is empty you can join, but if the server is full & someone with a reserved slot whats to join they they can & someone without a reserved slot gets kicked.
I have just mailed host about getting a server over 24 slot.
Next Tuesday this server will be a 24 slot. If I up it now I will be paying £10 just for 7 days use as they don't take part payments.