The good thing is that you don't have any problem if your mother do any affairs.
things are different for different people so cant say she need sex or not once she passed to 50 age.
As you mentioned that she first get widowed and then her second marriage is also end, she might be feeling lonely.
and as you are now 20+, that doesn't mean that your mature enough to tell her to do that do this. you can't force her to do affairs, etc.
so things you can do -
you can have a conversation with her about her being lonely. if she responds positively then you can continue on this or else you need to stop this discussion.
and about sex. so you asked does she needs sex more or less.
well we seek someone so we can talk with them, share our thinking with them. it can be anyone, like our friend our cousin etc. so basically we need a person to share our thoughts etc. that's why people married ( most of us get married coz of our parents). anyway, sex is important for every human being, but it's not must do thing in every relationship.
for example, suppose you introduce her to any friend of yours or any other guy, she is lonely now, with him she might go to movies, to shopping,to dates etc. but it isn't compulsory to have sex with him.
I hope you understand what I'm trying to say till this point.
thing is that you know that your mother is lonely. but you can't suggest her to do affairs and have sex directly. and you shouldn't make that approach.
Instead you can talk with her that you see her being lonely, if she wants to get remarried or not, etc.
any doubts then ask me here. and keep us updated