the answer really comes down to Raid Group sizes, not sure if this is a 7-mode system or not, although that doesn't neccesarily matter.
assuming you are just looking to add SAS disks, what happens next will depend on raid group size (this can be seen in the aggregate config).
Aggreagates are built up of Raid Groups - and personally i make my RG sizes a logical size, so in a system like yours which is a 24 bay chassis with 24 bay shelves, i would probably make my raid group size 24, which would mean a complete RG would fit in a shelf.
it is RG's that dictate the need for more parity drives which is why RG's are important here.
for example if you had a 12 disk RG size in your setup, your aggregate would be made of 2 raid groups, each RG (assuming using RAID-DP) would habe two parity drives in it.
one those RG's are full, the next drive you add will create a new RG. so if you have two full ones, then the next RG will need to consist of at least three drives, however only one of those would contain data.
If you RG size is bigger than 24 (up to 28 i think on your 2552 with SAS disks) then you could add another 4 SAS drives into the aggreagte without needing a new RG - so they would all be additional data drives.
so to try to answer your questions
1 - will depend on RG size
2 no - you can continue to grow the aggregate up to it's maximum size, as aggr can contain multiple RG's - you just need to understand the balance between resillience, performance and the impact of very big aggr0 (which i assume in your case contains vol0 in the SAS aggregate).
3- as per number 1 - this will depend on RG size.
be worth a look at the disk management documents for your platform and version of ONTAP so you can get a full understanding on RG's and thier impacts - rather than my no doubt confusing explanation!