You will need to take the following into account:
What's offered and the prices of these have varied slightly in different years, and it can depend on sponsorship and ticket demand. With very few exceptions, tickets sell slower and later than you'd hope.
One thing to keep in mind is that sometimes people want tickets but need permission from their employers if it's a work expense or they need the time off. So, encourage people to have those conversations sooner rather than later.
Lately (and wisely) this has been run as a ballot system. Another option is to use a tool like Travis Foundation's Diversity Tickets. However you go about it, it's recommended to spread the word through as many avenues as possible to connect with people who might otherwise miss out on a ticket. For example, groups that deal with Aboriginal & Torres Strait Islanders, refugees, universities and low-income students, Rails Girls, etc.
Make the time to plan this out a bit, to ensure you're contacting these groups earlier rather than later, to ensure their audiences have enough time to apply.