Places, especially larger ones, create a major load on iNat infrastructure, so any project that involves the creation of a large quantity of new places might be challenging. 300 is a fair amount - if they are fairly small places, this might not be a huge issue. However, if they will be large, it might be best to check with staff to see if this is ok.
KMLs can have multiple shapes within them, but there is a max file size limit for kmls on iNat: