OP_RETURN lets you put up to 160 hexadecimal characters into the field.
Using letter count you get 48064 characters from a quick text grab of whatever you put in the OP
So 48064 / 160 is 300.4 so 301 transactions.

IIRC once you use OP_RETURN the coins you send can never be spent again. (The rest of us thank you for making the BTC we have more valuable)
So you are going need to be careful with your inputs.

Take a look at:
https://blog.trezor.io/creating-permanent-uncensorable-messages-on-the-bitcoin-blockchain-fdbcb229732d

So if you can get them all into the smallest transaction and only have inputs that are the minimum size needed for the fee and the transaction size figure a few thousand USD at this point.
Although my math my be WAY off.
And nobody is going to be able to read it anyway, unless you are 100% sure you get all the data properly in consecutive blocks. Otherwise it's going to be random text.

Probably better off going for one of the crap alt coins that allows much larger fields.

-Dave