It depends a lot. If you do it a lot ( or he does it at nursery)
Dd2 picked up singing the alphabet very early, before 18 months, from dd1 singing it. People would stare at her as she sang it. Illusion somewhat shattered when she asked "What does el-emnopee mean?" (say it out loud!)
I don't remember her counting, but I do remember discovering that she knew her numbers really well when I said we needed 7 of something over round about her second birthday. I bought 3, and turned round to find her holding 4. So I said "is that 7, shall I check" and she rolled her eyes and said the toddler equivalent of "don't be stupid of course it's 7". I found that she was totally confident of doing that with any numbers up to a total 10.
Dd1 didn't sing her alphabet until she was at preschool at 3yo, but she knew all the sounds of her letters by around 22months and was reading and writing by 2yo. She had much less interest in singing songs and much more in letters and numbers individually.
She learnt to recognise numbers one grotty day when she was just under 2yo and we were waiting for a delayed number 2 bus. It was delayed by an hour and we saw every other number go past several times in that time. By the end of the hour she knew her numbers!
Ds I don't think I've ever heard him sing the alphabet and he's 9yo. Really not interested in such things. However I'll maintain he is probably the only toddler to be able to read and spell "Concorde" and "Sonic Boom" before their own name (and it's not a hard one). He needed those words to find the you-tube videos he liked, so he learnt.
He must know the alphabet though as he can put things in alphabetical order. I've never thought of that before!
For small children it's a mixture of what they like and parental approval. That's not saying the latter is a bad thing. if they've found something they find fun and they get parental approval from they want to continue, and that's great.
just bear in mind that ultimately what they need is not to be able to recite their numbers of alphabet, but to recognise the individual ones and be able to do 1-2-1 mapping for numbers to objects. Many children initially count by reciting the numbers as they point to the objects at a totally different time frame. Things like stairs are great for that as you move speak, move speak.
Although when dd1 first counted she counted with even numbers because we took it in turn to count down the stairs every morning. 