NO, it is per person only. And the whole variety is easy to see, of course also in /en/ on the site of the urban operator, mivb-stib. It is slightly cheaper for MIVB (being dutch, I stick to the dutch/flemish versions) only, vs also ON the territory of the Brussel-agglo-meration, other flemish (deLIJN) and walloon (TEC) regional buses. I am not sure how Diegem fits in it, have to check.
Also note that of course fastest to/fro airport is the national train, NMBS/SNCB, but any exit/entry at the airportstation also requires an extra airport Diabolo-surcharge of over 5€/time=much more as a normal single would cost. There are several local buses too-without this. Or perhaps the place you stay is closer by another local smaller station.
IF only using MIVB easiest and even cheapest is just check IN with bankcard (yes, british will also work), it is 2,30 per ride (incl normal changes), but topped at 8,40 per calender day, taken in the night after.
MIVB runs the metro, trams and citybuses, including a few routes fanning out of the city. If more is wanted or also other means/operators: check their site.
There are reductions for seniors >65, but these require additional administration and a personal MoBiB chipcard, so not really worth it for just 1 day.
