A team normally needs for a whole season as average between 6 and 8 tubs.
They carry 4 at each race (3 assembled cars, official + T, and then a spare tub disassembled) and then need a few more for testing.
The limitation on testing, with a max of 2 cars used contemporarily each day also reduced the total number of chassis needed compared with few years ago.
The first tub built is used for crash tests and typically not used for races, possibly only as spare for the very first races when further tubs aren’t ready yet.
Usually teams build pretty much all the chassis they are planning to use at the start of the year so by late April they stop the “production”. At that point only in case of irreparable damages or in case of modification of the design/building method, new tubs are needed.
Amongst the theoretically identical tubs that teams have since the start of season, you’ll find that more often than not, a driver will show a preference for a given chassis and, although with varying level of affection, will try to use it as long as possible. During the whole season, bar exceptional circumstances a single driver typically uses a couple of different chassis in total for the races and more often than not one of them for lot more races than the other one (something like 13-15 vs 5-3).
Furthermore, contrarily to few years ago when #1 driver of each team often was using 2 cars in practice to then pick for the race the one he preferred, nowadays the car you make the first lap of the weekend on is the car you’ll keep till the end of race hence they don’t change chassis but in case of damage. Given then the engine rules, the number of laps in the weekend is drastically reduced (Renault typically didn’t even make a single lap in the first hour of Friday practice). Nowadays in a single weekend drivers run 500 km or little more, considering the whole season this means 9000-9500 km; a tub can easily cope with that, it would be quite worrying if it wasn’t the case.
Consequently that Alonso apparently used the same tub for all the races isn’t something extraordinary and tells nothing about Renault budget (that is lot bigger than Briatore would ever admit, even under torture... more or less 2.6-2.7 times more than the 1/3 of biggest one

), reliability, ability to build cars etc etc. probably if you looked at the km each one of the existing R26 tubs completed on track in total you would find that the R26-03 isn’t even the one used the most.
Very likely it was Alonso himself that preferred it over the other ones and not necessarily because it was better, possibly just because he, consciously or not, convinced himself that it was the lucky one.
manchild wrote:
FIA scrutineering reports can confirm that.
In the scrutineering reports the cars are identified only by race number and the race number is related to the driver. Chassis number isn’t reported.
The only thing the scrutineering reports can confirm is that Alonso never changed chassis during parc fermé because that would be indicated (as for example Massa in Australia), but you can’t find there if the tub used in a given race is the same used in other ones. Actually it wouldn’t even be reported if someone changed the chassis while not in parc fermé, for example between Friday and Saturday, it would be just like changing any other part, allowed freely during the weekend bar in parc fermé when approval is needed. Only “special” part the change of which is always reported is, obviously, the engine because of the related penalty.