The number of engineers required for facilities depends on the number of similar facilities that have been previously built. (It appears engineers in the new XCOM are capable of working on an infinite number of tasks simultaneously, provided the tasks are sufficiently different from one another.)
Six engineers are required to build the first workshop, fifteen for the second, and ten additional for each additional workshop, the number of scientists required per laboratory is similar.
For satellite uplinks, the formula is (10 * SU + 15 * SN), where SU is the number of satellite uplinks already built (or under construction), and SN is the corresponding number of satellite nexuses. That is, you need ten engineers per satellite uplink, with the first uplink (already built at the start of the game) being free.
For satellite nexuses, the formula is 5 + (10 * SU + 15 * SN).
Satellite Uplinks vs. Satellite Nexuses
If we ignore adjacency bonuses, as nexuses provide double the uplinks for half again the number of engineers, this means the engineer cost of satellite nexuses is lower per satellite.
Factoring in adjacency bonuses, the upper limit is four satellites per uplink or six per nexus: each facility can particpate in four adjacency bonuses, but each adjacency bonus is shared between two facilities, so we get maximum two bonuses per facility. This 4:6 uplink ratio equals the 10:15 ratio of required engineers, so an infinite plane of satellite uplinks would provide the same number of uplinks per engineer as an infinite plane of satellite nexuses.
Since we'll never come close to approximating an infinite spread of satellite uplinks, in practice, nexuses will always have a lower engineer cost.
However, since they require alien materials to construct, and since having more engineers lowers the quantity of both money and alien materials needed to build everything, if you're short on materials, it's better to aggressively push the number of engineers, and build regular uplinks, saving the alien materials for armor and weapons.
If, on the other hand, you have a surplus of alien materials, the lower engineer, power, and space requirements of satellite nexuses provide more uplinks faster, and cost fewer credits.