As to RCD in series, maybe wrong word, but looking at 100 mA feeding 30 mA and also with caravans and boats 30 mA feeding 30 mA. However point is it does not say you need two RCD's in a home, that is what people have interrupted it as saying.
As to earth leakage, 3.5 mA is the limit for normal equipment, and a RCD should trip between 15 and 30 mA. So the limit could be exceed with just 5 sockets, clearly not every item will be to the limit, but also having a 100 sockets on one RCD is clearly over stepping the mark. As to what can be considered reasonable hard to say.
This house has 16 areas/rooms 3 will not have sockets as contain bath or shower, so 13 rooms which includes the hall and landing. Not got the guide to hand, but seem to remember 4 sockets per room, and more in the kitchen, so reasonable to expect 60 to 70 socket outlets, many will have items plugged in 24/7, kettle, washing machine etc. My house splits the sockets over 6 RCBO's if you include the socket on the cooker supply, all together there are 14 RCBO's (13 in use) only time they tripped it was correct, the flat roof leaked so socket got wet. However old house with just two RCD's it seemed to go in batches, could last a year with out a trip, then would get 5 trips in a month, then another year, no faults found, as it if spike on the line, or what not a clue, and resetting one could cause the other to trip. Maybe it was their age, fitted in around 1993 when my lad started to study to be a radio ham.
As to danger yes clearly a danger, I have noted RCD tripped and reset it, and have not told my wife, as far as I knew it had just tripped, then find some thing in freezer which looks as if it has defrosted. Today the freezers show the highest temperature they reached during a power cut, if you notice it before opening a door which cancels the display, and with the RCD in garage or granny flat it can be an occupant of the house can't reset the RCD as no assess. So possibly no heating.
In the main the house was wired before we started using RCD's so no consideration has been made that for example it feeds a shed or outside light which may cause it to fail. Often there is simply no consideration, we fit a consumer unit with two RCD's and it really does not matter where they feed.
I had mothers house rewired, the kitchen had a mini consumer unit fed with SWA, the guy wiring the main consumer unit was going to take it from one of the pair of RCD in main unit until I said something. Even then it seems he did not use the normal split even for the bits he had rewired, it was more down to what was easy. So when the overhead supply to garage went down, lost all but kitchen sockets.
I returned to UK around 1992 and all I heard was 16th edition says this, and it was also the first time I had come across the RCD, it clearly could have saved lives specially when some one knocks a nail through a cable to hang up his coat, but that took out the 30 mA, 100 mA, 1 amp and 5 amp RCD the last three all with time delay, it took out a huge chunk of the site, but was why I fitted them at home. Two RCD's fed two old Wilex fuse boxes. And with them been fitted for around 27 years, I have got use to them. And likely did safe my son who would play with electrics, now an electrical engineer.
But when I fitted them no rules said I should, and in hind sight two was not enough, but fitting after the fuse box was not easy. Even today with CU being split the electrician often has no option but to fit just two, as the box is not designed to fit 3 or 4 unless using RCBO. But with the reduction in price of the RCBO it is not so cut and dried, clearly in a caravan we fit one RCD, so to say a home must have x number RCD's seems daft, it clearly must be down to size of the home, but I would still say for an electrician to fit just 2 in my house would be wrong, and the same with neighbours houses. It needs at least 3, maybe 4, so lights in any room are not on same RCD as sockets in same room.