Hello. Please allow me to clarify a few things, and I do apologize for the brevity and lack of clarity in my original response.
First, there is no rule or best practice that states you have to keep the same number of disks per controller. Your workload, business requirements, business practices will dictate that. I stated in my original response that "it looks like you're going to split the disks evenly..." because that was implied in your original post. At the risk of muddying the waters further, I would say, if it were me, and only allowed to purchase 6 to 8 SATA disks (where I only had SAS before), I would create just one large aggregate on only one of the controllers as that would be a more efficient use of your limited number of disks. This would give me better disk performance and more useable space. To give an example (using raid-dp): if I had 8 disks, I would create one 7-disk aggregate, where 5 of the disks would be data disks (and 1 spare disk on one controller and 2 parity disks). If I created two aggregates (one for each controller) I would end up with two 3-disk aggregates and only a total of 2 data disks (2 disks used for spares, 1 per controller, and 4 disks used for parity). But like I stated before, your business needs, workload, etc. would dictate what you could do.
Second, to clarify the parity issue... using raid-dp (which is NetApp best practice), you need 2 parity disks per raid group. So the minimum number of disks you need for one aggregate is 3. If using raid-4 (one parity disk), you can create an aggregate with a minimum of two disks (one data and one parity). The reason I stated you would need 8 disks (4 per controller) instead of the 6 you mentioned, is because you implied raid-dp and stated your existing drives are SAS and you're adding new SATA drives. This means that you will need 1 spare SATA disk per controller (or Ontap will get very upset).
In terms of future addition of disks, you can add disks one at a time (subject to availability of hardware slots and room in the raidgroup size), although I must confess I know no one who buys/adds one disk at a time. When you grow your existing aggregate by one disk, be aware that you could subject yourself to performance issues and may have to do a reallocate to balance the data in the aggregate. The NetApp System Manager software recommends adding disks to aggregates three at a time minimum. I don't know if it still does that with the latest version.
Lastly, I am unclear about your question on how this all affects thin-provisioning a lun so I will defer giving any opinions there. Please feel free to ask for clarification if I succeeded in confusing you more.