By save, I guess you're meaning cache and not serialize it as an object instance. Anyway, with all the form APIs floating around, it's hard to make a judgement. If you're using pocketmine's Form class, feel free to reuse the same instance.
pocketmine\form\Form itself does not have any restriction on using the instance, but there might be other effects depending on the forms library you use. In particular, there may be race condition if you send the same form to the same player several times.
The form is being used to teleport to places. I also plan to have a good 35 players, so I don't know if creating a new instance or reusing the same instance will have any impact on the server performance because when you join the server you have to pick from the form.
Unless you are doing it like 100000 times per second, creating new objects has no impact on performance.
In case anyone is really paranoid, 100000 is just a random number I typed. Don't take this number seriously and count how many times you construct objects and compare with this seriously. In short, don't premature optimize unless you are sure it has a problem or if there is a very simple fix that does not affect more than 2 lines of code.