Scrimmages are literally the most important thing you can do to help solidify your players’ understanding of your formation/system before the start of the season. There are just no easy substitutes. I recommend a minimum of two before the start of the season. The biggest gains your team will make will be during the first few games together. Why wouldn’t you want to reap those benefits before the season starts?
The problem of course is that you never have a full practice field to yourself and the teams you share a field with are likely not to be the same division. All I can recommend is that you get the email addresses of the other division coaches and get creative to make it happen. If you can make it happen without consuming your entire practice even better.
During these scrimmages, your #1 objective should be the players positioning and how they are playing their positions. That means not only are the players holding the space/shape they should, but are they lining up against the correct opponents and are they taking good calculated risks for the positions they’re in. With that last point, I simply mean does that fullback take the opportunity to step in front of the forward to destroy the play vs does your centerback delay correctly. For 10u and up split the duty with your assistant and take notes–either during the game or right after. Those nuanced observations will vanish by the next day.
The value of scrimmaging declines greatly after the season begins. The players will have gleaned the majority after the first 3 games/scrimmages. So additional scrimmages simply means you are taking touches away from them by playing full sided games instead of playing small sided games where they will develop faster.
Note: For liability and insurance reasons AYSO coaches are required to stick to their assigned fields and times to conduct scrimmages.