So you've made it past whatever red-tape process you needed to upgrade or put your first LMS in place? Chances are to get to that stage you've already had to put up your case and fight for it to get it this far. To be successful buy-in is an absolute must so you're obviously doing something right! If you haven't got it this far... well... okay, we'll cover that next time!
The next problem is that you've now got a number of interested parties who all want to get involved and from the start you have to set the tone for the whole implementation if you don't want to find each decision and step is made by a committee of well-meaning but ultimately inefficient people. The key is not in keeping everything to yourself - you're definitely going to need help and further buy-in, but in keeping the implementation on-track and able to make key decisions. If not, you're going to get project creep, budgetary issues and a whole can of worms that you won't be able to get the lid on.
The most important thing to understand if you're running the implementation internally is that it's your show and you have to own it and bring people in and control the process. If you don't someone else will if you're lucky or it will be mob rule if not. Their is literally no limit to the number of people you can involve but the more you do the more time you need to allow and when you're with the vendor or partner that you're using to implement the system you need to remember that time is money and you can't afford to turn up with everyone with even a passing interest in your system. If you want widespread involvement then have a number of meetings with different people and go to all of them, gather up their thoughts, ideas and desires for the new system and bring them with you or have a representative from those meeting at yours.
What's the number we're looking for here? Well, when I'm running a configuration workshop for Totara LMS with a new client I'm ideally looking for a max of about 6 people from an organisation. I've done it with 1 and that's great if you're the only one with the drive, but for bigger organisations that's just not practical. The perfect number depends on the size and functions being represented, but I'd say a representative from IT, L&D (hopefully the lead and at least HR if not) and the eventual systems administrator (possibly your training coordinator or the like) is a pretty good start. Maybe you have someone who is effectively the sponsor of the project (your senior rep) and another one who is your 'trainer' or training designer to round out the group. 5. Yes, that sounds good because you have an odd number to get good decision making too! You can have a wider group to start where stakeholders can join in - but keep that as the opening meeting and then kick them out and get down to business when you start needing to make the decisions around the system.
Again, I'm not suggesting groups like marketing and comms don't have input, they clearly do particularly around look and feel; but you don't want them contributing too much into the workflows for development plans or your appraisal processes. In fact, that's a great strategy - use smaller groups for specific functions but don't keep bringing the wider group in for every part. IT are an absolute must (yes, must) and you want to engage them early, but past the technical they won't want to know and will get bored real quick if you bring them to everything.
Finally, remember that too many cooks spoil the broth, too many chiefs means no work gets done, too many people means that there's never enough chairs/biscuits/coffee and most importantly decision making suffers as the number of people increase.
(I have in fact ran a configuration and training session where I was the only person present, whilst I strongly believe in a minimalistic and agile approach, this takes the theory just a little too far... that organisation is still struggling with getting people trained and a lack of configuration two years down the track unsurprisingly)