While there are successful cases of conferences being held where tickets are not required to be admitted, We do NOT recommend doing this. (TODO: elaborate)
Tickets need NOT be paid. This is a very sensitive area that will need much consideration by each organizer, as this is directly related to multiple factors such as the main topic of the conference, the social/financial situation of the target audience, and many others.
Please note that the decision to sell tickets and to make it free or not is ultimately up to each organizer.
If you are using tickets to track the number of attendees, make sure to note that you will ALWAYS have a significant number of no-shows. This will again vary greatly depending on what your conference is about and your attendees. But regardless, you will see more no-shows than you ever expected.
A peculiar example is conferences held in the Tokyo area. Tech conferences in Tokyo has a very distinct characteristic in that a massive majority of the attendees will be from the greater Tokyo area. This is because most of the tech population in Japan is concentrated around in that area. An advantage caused by this fact is that it is extremely easy to gather enough people to hold relatively large conferences in Japan, because almost everybody who will be your target audience live within 1.5 hours away from your venue. The downside is that it is also extremely easy for these attendees to cancel their attendance. So compared to conferences where a significant number of your attendees must commit to air travel, conferences in Tokyo area will have a sigifnicantly higher no-show rate. From experience we tend to assume that 20% of the ticket sales will be no-shows. Again, this will vary significantly depending on where/what your conference will be held.
The point is that you should plan your capacity with no-show rates. For example, if your venue's capacity is 1000 people, and your no-show rate is 20%, you should plan to sell around 1200 tickets. Do note that you should also be checking the number of order cancellations, and plan accordingly. You should probably try to oversell your capacity, but you also must make sure that you don't make your venue overcrowded.