Let $I$ be an ideal and let $I^+$ denote its complement (the so-called $I$-positive sets). Now we say that $I$ is $\lambda$-saturated iff each antichain in $I^+$ has size less than $\lambda$. Further $sat(I)$ is the least cardinal $\kappa$ such that $I$ is $\kappa$-saturated.


It can be shown that if $sat(I)$ is infinite then it has to be uncountable. I think that a similar argument gives us that $sat(I)$ is a regular cardinal, provided that $I$ is a $\kappa$-complete ideal on $\kappa$.


My question now is: Do we need the $\kappa$-completeness of $I$ or is $sat(I)$ always a regular cardinal, no matter if $I$ is $\kappa$-complete or not.



Is $sat(I)$ always a regular cardinal?
You do not need the $\kappa$-completeness of $I$. In fact the following holds for an arbitrary partial order and is exercise F4 of chapter VII in Kunen's book Set Theory: an Introduction to Independence Proofs.


Theorem. (Tarski) Let $\mathbb{P}$ be a poset, and let $\kappa$ be the least cardinal for which $\mathbb{P}$ has no antichain of size $\kappa.$ Then $\kappa$ is a regular cardinal.


Here we consider elements $p,q$ of $\mathbb{P}$ incompatible if there is no $r\leq p,q$. So $sat(I)=\kappa$ for the partial order on $I^+$ ordered by $\subseteq$. 


