This is very dependent on your geo location (what country and state you will be seen as operating from). Here we will provide you what is needed in USA just as an example. Do your own research to expand on what we have here and maybe get a business lawyer to look at it at some point.
The money exchange business needs a series of licenses and regulations. Normally these norms vary from state to state. Not having the required license is considered to be a criminal violation. Each state has a separate licensing department and individuals interested in starting this business, need to apply for these and get the required nod before proceeding any further.
The other associated step is paying up the required fees in this aspect. This again is a state regulation and dictated by the individual requirements of every state. Payment of these fees to the right authorities prevents unfair trade practices and corruption possibilities. Also this helps you conduct your business in a peaceful manner without disturbances and interruptions resulting from sudden check and resultant penalty service.
The basic operating procedure involves obtaining a Registration of Money Services Business application and submitting it to the Financial Crimes Enforcement Network see https://www.fincen.gov/. This is a standalone agency within the US Treasury Department.
You will also note in this thread https://www.fincen.gov/resources/statutes-regulations/guidance/interagency-interpretive-guidance-providing-banking that the threshold to even require getting a license as a Money Service Business is transactions of more than $1000 USD per day to any single person. I didn't see any limit on the number of people you can deal with in a day but in other documents I see a threshold of $50,000 USD per year mentioned.
“Under existing Bank Secrecy Act regulations, FinCEN has defined money services businesses to include five distinct types of financial services providers and the U.S. Postal Service: (1) currency dealers or exchangers; (2) check cashers; (3) issuers of traveler’s checks, money orders, or stored value; (4) sellers or redeemers of traveler's checks, money orders, or stored value; and (5) money transmitters. There is a threshold requirement for businesses in the first four categories – a business that engages in such transactions will not be considered a money services business if it does not engage in such transactions in an amount greater that $1,000 for any person on any day in one or more transactions. See 31 CFR 103.11(uu).”
From Brayan M: “Thanks sacarlson, I copied the info above and also Maryland's non-regulation document (http://www.dllr.maryland.gov/finance/advisories/advisoryvirtual.pdf) of virtual currencies and that it is not legal tender for future reference to inform members. Misinformation gets under my skin so this will be very helpful to correct members.” Note you should be aware of in that pdf file: CURRENTLY, MARYLAND DOES NOT REGULATE VIRTUAL CURRENCIES.
Form to 107 for Registration of Money Service Business for fincen: https://www.pdffiller.com/jsfiller/?projectId=118033951#41b91f23d1ea4d7dbf1a3a1a34bc05d0
In Thailand now they are becoming more accepting of crypto currency and have started publishing regulations that should make it possible to become fully legal at larger scales at some point. This article points out the beginning of that path. https://www.sec.or.th/en/Pages/News/Detail_News.aspx?tg=NEWS&lg=en&news_no=50&news_yy=2018
As stated each country is different. To get an idea of how different or similar it is here is a link that demonstrates what's needed in the UK to become a money exchange dealer: https://www.theguardian.com/money/2010/dec/11/foreign-currency-dealer. Many factors look to be about the same just different forms to fill out at different agencies with different fee's and costs.