There are two basic doses of Tadalafil.
20mg is the "event" dose - i.e. take before sex. Tadalafil lasts much longer in the body than Sildenafil, so the 20mg dose often still works next day too. You are unlikely to get more than 2 per week prescribed (and for ED, many doctors limit to once per week which used to be the limit when there were no generic versions, but no longer is provided you're not being prescribed branded Cialis.)
The other basic dose is the 5mg daily low dose, which is aimed at men who would need the "event" dose more than twice a week.
In the case that either of these produce unacceptable side effects, they are also available in half doses, 10mg and 2.5mg respectively, although these half doses aren't any cheaper than the full doses.
Now this is all for ED (erectile dysfunction), i.e. improving diminishing erectile function.
For prostate cancer patients, there's a secondary use of Tadalafil, and that's for penile rehabilitation. This applies both in the period after nerve sparing prostatectomy, and also while on hormone therapy. The evidence that Tadalafil works for this is a bit limited, but there no evidence any of the other PDE5 inhibitors work at all for penile rehab, and it's very likely they don't because they're much shorter acting. The purpose of this use is not so much to generate erections but to help protect the tissues of the penis in the absence of erections, and in the case of nerve sparing prostatectomy, to improve blood supply to the nerves to help with healing.
The dosing used for penile rehab is usually the 5mg daily low dose, or 2 x 20mg/week. An advantage of the 2 x 20mg/week dosing is you are also getting an event dose twice a week which might enable you to get an erection, whereas the 5mg daily low dose is less likely to be able to do that.
So in your situation, I would ask for 2 x 20mg Tadalafil per week, or if they won't give you that, the 5mg daily low dose. 2.5mg was the wrong dose unless it had to be reduced due to unacceptable side effects, or there was some other medical reason, but still much better than nothing.