It depends upon in which city you are living. However, Maruti Suzuki has a fully fledged manufacturing plant at Manesar which manufactures 5.5 lakh cars annually so there is a minimal demand-supply gap. So, the waiting period for Maruti Ritz is less than a month for diesel variants and around two week for the petrol ones.
Maruti has a good and vast service network and there will be no problem of maintenance and replacement of parts at later stage but the resale value of the car might decrease as the company has decided to discontinue the vehicle. Despite this, there will never be any shortage of parts as manufacturers make parts for next 10 years. Also, as Ritz share a good number of parts with its sibling Swift there will be no shortage whatsoever. If you would like to consider, the Indian auto market has many other offerings in the same segment such as Tata Bolt, Nissan Micra, Ford Figo, Honda Brio and Grand i10, Maruti Swift, Chevrolet Sail hatchback.