Darren made an excellent point with the amount of tunnels since if both are running in NAT mode and each side has 2 ISP's you can take your site count and quadruple that number to know your tunnel count. However maximum sessions is indeed another issue to tackle with. It must be that you have too many hosts each making small concurrent sessions. In that case if you want to keep the SD-WAN design you may need to change up to one-armed concentrators instead and group branches each to their own preferred concentrator.
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