The easiest way to understand how much load (watts) you can put on your lighting circuits is by understanding how load is calculated.
load = voltage x current
In your case you have a 5A circuit running on 230V so that is a maximum load of = 1150W or 1.1KW
You currently only have 3 x 75W = 225W of load connected to a circuit which can handle 5 times that.
By changing to 4 x 50W = 200W of load connected to a circuit which can handle 5 times that, so you have more than enough head room to add another 16 bulbs if you wanted to without any issue.
Please let me know if you need anything clarified.