To be honest I think your account would be safe even if you didn't disclose it was an advertisement. Incredibly famous celebrities are tweeting out sponsored links all the time and are not disclosing it with #ad or anything like that so I don't think you'd need to worry.
I think you should price your tweets for how active your followers are. Twitter is interesting because some people can have 5,000 followers and pull 100+ favs on a tweet while others can have accounts with 100k+ and barely pull 50.
So be honest with yourself and figure out how active your followers are. On average, how many favs/rts are you pulling on a tweet without a link? How many are you pulling on a tweet with a link? How many engagements are you getting on your tweets?
These are important things you need to be able to convey to your advertisers. Unless you are using a twitter publishing networking like sponsored tweets, you will want to build relationships with your advertisers as you don't want them to just purchase a tweet or two and not come back, you want them to be recurring and feel like they are getting value. Plus you need to actually be offering them value in the first place.
As for how many sponsored tweets per day, if you are interested in continuing to grow your account and audience I would say do no more than 1 per day. If your content with what you've built and just want to start earning maybe 2-3. Ad's do scare off followers, so be prepared to loose some either way though.
Many people delete tweets after 30 minutes because large accounts that sell promotion get most of their activity within the first 10 minutes of a tweets lifespan. If you plan on deleting the tweets after a certain amount of time make sure you disclose that to your advertiser and explain why you do it and how it benefits them.
You could even offer your advertisers packages, like 3 tweets spread out across 3 days, each staying up for 30 minutes.
And last but not least customer service. You can really set yourself apart from others by just being polite, direct, and respectful.
Good luck! Hope this helped!
