If you sign in with Twitter, you can search for a maximum of your last 500 mentions, and see who has mentioned you, and how many times. You can also read the tweets that mentioned you by account. For any other public account you can search for a maximum of 300 weets where the account is mentioned.
If you do not sign in, you can search for the last 100 tweets that mention any public Twitter account.
Please note Twitter only gives search results for a period of about 8 days, so even if you select a higher number of tweets, the results may be the same as when you select a lower number of tweets.