Hi
Good question. Selecting the right investors is one of the most important stages - as investors can make or break the venture. - especially if it's friends and family.

The amount of shares that should be given:
1. A good tactic in negotiations (and this is a negotiation) is to first try and find out what the investors want. It may just be that they were expecting a percentage that you are happy to give, in which case there is no problem (In this case, I would even give a little bit more than they asked for which will positively surprise them - even if it's just one %).

2. Calculating how many shares each founder should get:
There are various factors to take into account. These include: (a) the size of the investment (b) the current + potential value of the company, (c) the added value that the investor brings to the business (such as connections or expertise), and (d) how hard or easy it is to get investments from other sources.
At the end of the day, it's all about value and psychology (both side's feelings and perception of value).
Bottom line:
1. It all really depends on how much value they are giving you (not only financial, sometimes even just moral support goes a long way).
2. Ask the potential investor how much shares they want (before YOU give a number).
3. Have an open conversation with them in regards to each of your expectations.

I've successfully helped over 350 entrepreneurs, startups and businesses, and I would be happy to help you (including drafting the investment agreement if need be). After scheduling a call, please send me some background information so that I can prepare in advance - thus giving you maximum value for your money. Take a look at the great reviews I’ve received: https://clarity.fm/assafben-david