There are two basic doses of Tadalafil:

20mg which is the 'event' dose, i.e. intended to take an hour or so before you want to have sex.

5mg daily low dose, which generates continuous dosing so you can have sex without pre-planning, or more often than you can obtain or take the 20mg dose.

Both of these also come in half doses (10mg, 2.5mg), which are intended for the case where the full dose gives unacceptable side effects. There is no 50mg dose.

Tadalafil lasts quite a while in your system, at least, compared with the other PDE5 inhibitors. For many people, this means a 20mg tablet still works a day later. When you are on the 5mg daily low dose, the level in your body each day is equivalent to 8mg, because some runs over from the previous day.

Thats's all basic ED usage. For prostate cancer patients, it has a couple of additional uses too.

Tadalafil opens up some blood vessels in the body, generating higher blood flow. This applies in the penis, even when it's not erect, and some other tissues around it. This may help nerves to heal faster after a nerve sparing prostatectomy. It may also help avoid damage to the penis during a period of being unable to get erections (e.g. just after prostatectomy, or while on hormone therapy), since regular erections are normally necessary to keep the tissues of the penis in good shape. For this purpose, you want continuous dosing, which can either be done with the 5mg daily low dose, or by taking 2 x 20mg tablets per week.

My surgery would only prescribe me 20 mg Cialis twice per week. The tablets are easy to break in half so I take 10 mg every other day. As Andy says, with the longer half life you will still have some in your system the following day, so almost as good as 5 mg daily. It does tend to give me acid reflux so I would rather avoid the event dose. That said, the 10 mg plus cock ring is working very well for me.