I would recommend one. I thought I wouldn’t need one so I didn’t buy one and it didn’t take me long to change my mind. I got a pack that came with two from Amazon (not wham bam) it was $15 for two. If I’m printing back to back I take the flex plate off and put on the other one and start the printer while I go and do my post processing. If you are making smaller and more delicate things this also makes it much easier. I have the Elegoo Mars 3 pro but I am going to be upgrading to the Saturn 2 also and plan to get new flex plates to fit the new printer

In case you are wondering this is the one I purchased. They offer other sizes as well.

https://www.amazon.com/gp/aw/d/B09MQSRZ8W?psc=1&ref=ppx_pop_mob_b_asin_title