Number of people in your safari group

The standard number of people in a mid-price safari minibus in Kenya is six. This means that everyone will have a window seat on the bus and will have access to the roof hatch. You need to check that your chosen safari actually says "All clients have a window seat and access to the roof hatch": some cheaper companies pack eight or nine people into a minibus. Some companies have seven clients per bus "all clients have a guaranteed window seat" - this drops the cost a bit, but one person is on the front window seat, next to the driver, but without access to the window hatch. A good driver will try to arrange it so that that client is at the side where the animal or bird being watched is on their side, but this isn't always possible. The front seat should be rotated between the clients.
On this sort of trip, you can get lucky, and find that there are fewer clients than the maximum, especially off-season. You may find that there is a 'minimum clients' clause - usually four - without which the safari will be cancelled. You may get short notice of this.

Be aware that 'minimum number 2' means just that, not that the tour is limited to two. Generally, if you buy a fixed itinerary, you are buying a seat on the bus. Read the small print carefully. but you might get lucky!

A group is seated together at meal-times in the lodges, so if you clash with your fellow-travellers it could be difficult, as you'll be with them a lot of the time!
In Tanzania, although some mini-buses are used, it seems much more usual to use Land-cruisers or equivalent. In these, I would think four would be the absolute maximum for comfort and manoeuverability inside, but I saw six clients plus driver often.

The best - and most expensive - option is to arrange a private safari, for many reasons, but it obviously puts up the price of the safari a lot, since all the expense of the driver and vehicle have to be shared amongst all travellers. However, depending on the company you choose, it won't necessarily be that much more exppensive, and it could be about the same price as a 'guided tour', as you're not paying the guide's expenses and wages. Conversely, if you have a non-resident guide travelling out with you, that will add to your cost, as your party has to cover their airfare, single room (probably) and salary.