how to knit flat on large project on multiple circular needles

I have an expanding project that no longer fits on my longest circular needle. Can I use multiple needles? It is just a flat knitting project.

Yes, you can use 2 or more circulars. You only have to remember to knit with the matching needles in each section so that you don’t accidentally transfer sts from one circular to the next.

I would disagree with keeping sts 1-10 on first needle, then 11-20 on second (for example).

I often use two circs and yes, I do change what’s first and last each and every round. I’m knitting rounds and it’s easy and for flat I should think something like right side

K all of first ndl, and 3 of second onto first (now you have 3 sts more on first ndl)
next row something like when it’s time to change needles, again go 3 sts from the next needle to the one you are using.

Pay a little attention and no, you won’t get lost. Use markers. Feel free to make it 2 or 3 or 4 or how many sts but I’d do something like 3 and 2, turn and do 2 and 4. Yes must pay attention to your pattern and get it right.

What I’m saying is I never change needles in the same places (after certain amount of sts) every time. Change it and there will not be any looseness.

Try it with a much smaller number and back and forth for a few rows and see what works for you.