If you´re dependent for more than 75% of your income from one client, then you need to formalise your working relationship in a specific contract and register your contract with this client with the State Employment Agency.
There is a special ´regimen´ for those Autonomos, called TRADE.
The TRADE Autonomo has the obligation to present the contract within 10 ´días habiles´ and after that, inform the client within 5 days.
If 15 ´días habiles´ have passed since the contract was signed and the TRADE Autonomo hasn´t communicated that he or she has registered it with the State Employment Agency, then the client is obliged to do so within 10 ´días habiles.
Link to what exactly are ´días habiles´.
More info about TRADE, in Spanish,
And from the Government website
Scroll down to 6.3.3.