Aelis, Europe is big continent! For a first trip, it's less a matter of how "how many places we can see" as much as which of them are going to suit your interests at a comfort level with language, transport and whatnot you are willing to take on. The worst thing you can do, IMHO, is to try and cram so much into so little that you don't have time to learn or recall much of anything at all! You might spend most of your time (and money) just packing up, moving on, unpacking, trying to get your head around a new location...
We've spent 21 days JUST in Italy and loved every minute..and that wasn't our first time. We hope to go back several more times for a month or two! Other RS members will say that same about France, Germany, Greece, Spain, etc. What you need to do is to get some guidebooks, spend some time with online travel sites, and then decide which places have attractions/activities which most appeal to you.
Trip planning isn't everyone's thing. You may decide that it's more your style to sign up for a tour where most of the itinerary, hotel reservations and transport arrangements are made for you. Conversely, some of us - myself included - are independent travelers who are OK with putting a lot of time and research into creating our own trips. There's no right or wrong with either choice as long as YOU have a good time.
You might want to look at Rick's "Best of Europe" tour? It may be a good opportunity to get your feet wet with friendly company and support that will help you to feel confident enough to venture out on your own next time?