SQL Server 2019

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

(No column name)
Microsoft SQL Server 2019 (RTM-CU1) (KB4527376) - 15.0.4003.23 (X64) Dec 6 2019 14:53:33 Copyright (C) 2019 Microsoft Corporation Express Edition (64-bit) on Windows Server 2019 Standard 10.0 <X64> (Build 17763: ) (Hypervisor)

drop table if exists dbo.Results

CREATE TABLE [dbo].[Results](
    [MonthYear] [nvarchar](35) NULL,
    [MOnthNumber] [int] NULL,
    [YearDate] [int] NULL,
    [Clients] [int] NULL,
    [NonClients] [int] NULL
) ON [PRIMARY]
INSERT [dbo].[Results] ([MonthYear], [MOnthNumber], [YearDate], [Clients], [NonClients]) VALUES (N'February 2020', 2, 2020, 46, 0)
INSERT [dbo].[Results] ([MonthYear], [MOnthNumber], [YearDate], [Clients], [NonClients]) VALUES (N'January 2020', 1, 2020, 21, 2)
INSERT [dbo].[Results] ([MonthYear], [MOnthNumber], [YearDate], [Clients], [NonClients]) VALUES (N'March 2020', 3, 2020, 37, 0)

--unpivoting
drop table if exists #results
select MonthYear, MOnthNumber, YearDate, col, val, dense_rank() over (order by YearDate desc, MonthNumber) rn	--to preserve the order
into #results
from
(
	select MonthYear, MOnthNumber, YearDate, Clients, NonClients
	from Results
)t
unpivot
(
	val for col in (Clients,NonClients)
)upvt

DECLARE 
    @columns NVARCHAR(MAX), 
    @sql     NVARCHAR(MAX) = ''

-- select the column headers
SELECT 
    @columns = COALESCE(@columns + ', ', '') + QUOTENAME(MonthYear)
FROM 
    (select distinct MonthYear, rn from #results)R
order by rn

-- construct dynamic SQL
SET @sql ='
SELECT col as C_or_NC, '+@columns+' FROM       
(
    select MonthYear, col, val from #Results
) t 
PIVOT(
    Sum(val) 
    FOR MonthYear IN ('+ @columns +')
) AS pivot_table
order by 1 desc'

--print @sql

-- execute the dynamic SQL
EXECUTE sp_executesql @sql

C_or_NC	January 2020	February 2020	March 2020
NonClients	2	0	0
Clients	21	46	37