Hi, I was wondering what the recommend thickness for a foam cot mattress would be?

Are you looking for a mattress for a secondhand cot?
Are you referring to a household cot or a portable cot?
What age is your baby?

Remember a foam mattress may not be as firm as other cot mattresses & may not retain that firmness for as long.

Red Nose has the following information in relation to (household) cot mattresses.
A safe mattress is one that is the right size for the cot, is firm, clean and in good condition and is placed flat (not tilted or elevated).

Right size: Ensure the mattress complies with the size and depth recommended by the manufacturer of the cot. A baby or toddler can get stuck in gaps between a poor fitting mattress and the cot sides. This is especially dangerous if their face is trapped and covered, or their neck is restricted in any way. Make sure there is no more than a 20mm gap between the mattress and the cot sides and ends.
https://rednose.org.au/article/what-is-a-safe-mattress