Sildenafil is available in 25mg, 50mg, and 100mg doses.
Urology/Andrology often start at max dose, which isn't how drug dosing is normally tried. When asked why, the answer was they don't want to spend weeks trying different doses to find the one that works - it' important to resolve ED as quickly as possible for mental health. If a given dose works, then you can try reducing to see if that also works. Reducing in the case of unacceptable side effects is another possibility.
There's also a 20mg Sildenafil, but that's intended for treating pulmonary hypertention (the original purpose of Sildenafil). It's a mixture of standard immediate release Sildenafil and slow release Sildenafil, and is intended for dosing at 3 times a day to achieve continuous dosing. I have come across one case of surgeon prescribing it for penile rehabilitation after prostatectomy (but probably not 3 times a day!). I think it was because the local CCG wouldn't prescribe Tadalafil.