The only 'right' you have really is to be treated the same way as everybody else. They can't be unfair just because you are on mat leave, but you are not specially protected either.
FWIW, I used to work in this kind of area and there is a set process for doing it well, which it sounds like your company is following, so first of all that's good news in that they seem to be treating you fairly.
What will have happened is that the other employees will have been told this face-to-face this afternoon, then given appointments for their consultation meetings after the weekend. The manager then gets on the phone to people who are on mat leave, long term sick, holiday and gives them the same information and arranges a consultation meeting for them.
At your consultation you should be given lots more information on your options (without knowing how big your firm is it's hard to be specific) but these could include volunteering for redundancy or being at risk of compulsory redundancy (they should give you a figure for any payment), alternative employment - ie a transfer within the company, the number of posts being cut vs the number of people in place (eg if there's ten people doing your job they may want to cut five posts, but five people might volunteer for redundancy so there is still a chance you could keep your job). They should also give you a letter explaining your options, and you will normally have 14 days to consider what you want to do.
It is totally normal to be shocked and upset. Take some time over the weekend to think about it, also I don't know if you've considered what you wanted to do after mat leave but it's worth putting that on the table (e.g. if you wanted to cut your hours that may work for the business too).
Sorry you've had such a shock and sorry this post is too long! There's an employment forum for further advice but I just thought it might be helpful to outline what the proper process is, and as I said it sounds like they're following this which is good.