That is a good question and one that had never occurred to me before. I am not sure I have an answer to your question as to precisely how so many would be able to lay their hands on the head of the bulls. I would however, like to address one point you made as to the numbering of the Levites which may help you put this is a more manageable perspective.
In the numbering of the tribe of Levi in chapter 2, only the males were numbered from one month of age and upward. The total of the three Levitical families are recorded as follows.
The number of the sons of Gershon “of every male from a month old and upward…, 7,500.”
The number of the sons of Kohath “of every male from a month old and upward…, were 8,600.”
The number of the sons of Merari “of every male from a month old and upward…, 6,200.”
The total number of the Levite male population according to verse 39, was 22,000 from one month old and upward. The total number of the firstborn males among Israel was 22,273.
There is apparently, some discrepancy with the number of Levites given from the three Levite families. This is probably the result of some textual error in one or more of the numbers representing the three separate family lines. 2:39 has the total number at 22,000 which is correct but, 7,500 + 8,600 + 6,200 is not 22,000, it is 22,300, In which case the Levites would have outnumbered the first-born of Israel.
Verse 46 says the excess of the firstborn of Israel was 273. Each person over and above the Levites was to pay a five shekels ransom. Verse 50 says the total ransom collected was 1,365 Shekels; So, 1,365 divided by 5 is 273 which means that the total in verse 39 is correct. The error then, is somewhere in the recorded number of one or perhaps all the three Levite family lines.
There where qualifications that had to be met for a Levite to enter into the service of the tabernacle. They could not be blind, lame, dwarfed, or bald. They could not have crippled limbs, or crushed testicles. They could not be married to a woman who had been a prostitute or to one who had been divorced. They could not be younger than 30 or older than 50. This means that the number of serving Levites would be considerably less than then the 22,000. In the numbering in chapter four, only the qualified males from age 30-50 were numbered and that number totaled 8,580. Even still, it would have been a daunting task for 8,580 Levites to lay their hands on the heads of the two bulls.