Giving blankets to some or all?

I’d opt for providing enough for everyone. If you have extra that aren’t used, you could always return them or donate them, but I’d prefer to be over-prepared than not have enough.

I personally think that if you advertise that your ceremony will be outside, most people know themselves and know how chilly they get - and will wear a coat in accordance with that. I've seen a few posts on Reddit about this same question and most of the respondents there agree that blankets for 50% of your guests (or even fewer) should be fine as most adults can dress themselves for the weather. But you'll have some for older individuals, tiny humans, or people who are just naturally always cold! That's just my two cents, though.

Will the blankets be kept by the guest or will be they returned at the end of the evening? I think if people will be keeping them you need enough for everyone.

I would get enough for everyone. What happens when someone gets there, needs a blanket, but you've only gotten enough for 1/3-1/2 of your guests and they were running a bit behind, so they've all been taken. Also, I wouldn't expect an outdoor ceremony in the winter, so I would be wearing what I would normally wear to a wedding - and I am cold ALL the time. Is your reception indoors?

I'm getting married in December and its outside so we are providing blankets for all guests. It is a covered pavilion so we'll be safe from rain but I would hate for my guests to be miserable cold during the ceremony. We will move indoors for the reception.