It gets complicated. As I understand it, the Suns can have a couple of roster players in the G-league. As long as the players have no more than three years of service, they can be sent to the G-league. An NBA roster can only carry an active roster of 13 players with a maximum roster of 15 players. So I'm thinking the Suns can send members 2 of their 15 man roster to the G-League... maybe more. Then there are the two-way contracts.
The Suns currently have 4 draft picks. So we know Ayton is going to make the active roster and likely one other draft pick. That means the other two draft picks could be sent to the G-League. However, one of the picks could likely be a draft and stash player. Add in the two-way players, plus the other players that fit into different categories, i.e, expansion draft players, the Suns could have a healthy development league team.
The link below lists all the categories of players.
http://gleague.nba.com/how-rosters-are-built/