2. I'd shoot for 5 reference customers (paying list price, using it as you'd expect and genuinely delighted). Along the way I'd track who you try to sell to in a spreadsheet (including those that just drag on and never close) and I'd score them across the behaviors/things you would expect your target audience would have in common. This will help you create an Ideal Customer Profile - which you can then target more and more heavily (where you do marketing / what you build next and so on in future). Be _very_ specific not just ie by industry. For example, paypal targeted ebay power sellers in the early days.
3. Absolutely not. I've more than 40K customers at my startup and I live in a village no one has heard of in the UK.
4. Don't worry about scaling up via marketing until you have 5 reference customers (and generally do more of what got you the first 5 as your first step)
5. Read Secrets of Sandhill Road to understand the VC model more deeply. If you bootstrap - you have total control, might make more if you sell (because of how preferred shares work) but it will probably cause you more personal financial stress. Decide what motivates you - lifestyle business or trying to build a $10bn company. VC is an irreversible door, more or less, whereas bootstrapping isn't. I'd default with bootstrapping if unsure.