How easy is it to park in Rimini in August?

The whole of August is traditionally taken as a holiday for many Italians. This is less true now than it used to be due to economic conditions amongst other things, but still a good rule of thumb. This means that cities that are not typically holiday destinations for Italians are emptied and contain mainly foreign tourists, and seaside towns that are typically holiday destinations for Italians are packed out. Rimini, being the latter, will be full of Italians as well as foreign visitors. As such, street parking will be harder to find anywhere near the beach.

If you search a hotel booking site such as http://booking.com you can typically select required facilities such as parking. I did a search for one week in Rimini for the first week of August and restricted the search to 100-150 Euro/night and 3 or 4 stars. It's likely that any 5 star hotels will have underground garages or dedicated parking anyway. That returned 243 properites.

Selecting parking reduced that to 213. Although all results say Private Parking or Free parking, the details are typically given after selecting the hotel, e.g.

garage parking is available at extra charge

Private parking is possible on site (reservation is not needed) and costs EUR 16 per day.

Private parking is provided upon availability and is free between October and mid May.

Private parking is possible on site (reservation is not needed) and costs EUR 10 per day.

Secured parking Private parking is possible on site (reservation is not needed) and costs EUR 10 per day.

features a private parking garage

Free private parking is possible on site (reservation is needed).

Free! Free public parking is possible at a location nearby (reservation is not needed).

Private parking is possible on site (reservation is needed) and costs EUR 8 per day.

booking.com's FAQ states under parking:

How do I know if the hotel has parking facilities and how can I reserve it? Under ‘Hotel Facilities’ you can see whether or not the hotel has parking, and if any restrictions apply. If the hotel requests you reserve parking, please contact them directly with the contact information provided in your booking confirmation.

I don't believe a hotel could advertise parking and not have on-site parking or an arrangement with a nearby off-site car park. It would surely be a tripadvisor nightmare. However, they may advertise parking with no reservation and you find there is no space available.

A search on google maps for parcheggio in Rimini shows a number of car parks and parcheggio con parcometro, i.e. metered parking. At the worst case you would be likely to find a for-pay car park or on-street paid parking not too far away.

Rimini turism provides a list of car parks.
Comune di Rimni also provides a list of car parks.

A search for parcheggio gratuito (free parking) gives a tripadvisor thread indicating that free parking is available on the road parallel to the sea front. Obviously it's first come first served and those Italians can get up early!

As JoErNanO says, you are advised to contact the hotel and ask for their advice directly.